Pleached trees can be a very expensive mistake if you don't have the time to tend their needs for a year while they establish. You do see them so frequently looking folorn because people treat them as an instant solution. At such an exposed spot red robin will be a diseased mess...so many examples on this forum over the years.
That border seems not particularly wide either so make sure you are going to give enough water to whatever you plant there.
Personally I find it strange all this privacy demand in the middle of a sense housing development...surely that ship dialed when you decided to live there. It's such a weird British obsession my European brain can't cope with 😉
Lots of people have asked on here about problems with Photinia "Red Robin". Seems it's quite fussy about conditions, and looks rubbish if it's not happy, so check that you can give it the climate and soil that it likes.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
Most of the shrubs on your list will take a good 5 years or so to reach your desired height of 3m. Pleached trees would work but will be expensive and high maintenance.
Another consideration could be a row of multi stemmed small trees which will give you good height and spread once established. Rowans and Amelanchiers are possible options and widely available. Although you won't get a dense green screen like a hedge the view will be filtered during the growing season. You could use a mix of trees to avoid looking like an intended hedge and avoid the hedge height limit of 2m. I don't think you'll need more than 3 multi stemmed trees to screen off those windows and door. They will require regular pruning once they approach your required height though.
As you have a fair sized budget if needed, I would put in some substantial posts with a trellis slightly forward and maybe an overlap of the top height and above it. But I think slightly forward even just a foot or two as a hedges or trees would take up that sized space anyway at the least. And it would make for easier maintenance of the fence and the climbers. As it wold be a foot or two forward it would with the plants built up and billowing along the top makes for added height without too much space taken up or bulk. Sorry if it sounds confusing I hope you understand.
Or a few good sturdy obelisks with climbers would also work . You can get ones with an "umbrella" style top and ones with a kind of globe too I think.
Also consider that although you feel very overlooked, the distance of those windows means they will only possibly see your heads or a little bit more from that distance and height? If you look across from your upstairs windows what can you see on the ground or over the fences from your perspective? Although you look up and see them standing in their long window/door, with balcony thing, they might only be able to see you a little bit.
I have never seen pleached Photinia growing happily. Lovely at a Garden Centre or Nursery where it has spent most of it's life under glass. Hopeless in a windy site or on clay soil where leaf spot and wind burn can be an issue in winter. If one does die what next? The Obelisk idea is a good one if you want instant impact and something to draw the eye away from the fence. Also you can take them with you when you move. If your concerns relate to upstairs windows that is different. I have just moved house and the surveyor for the old property' was on the ball' with the plants the slightest sign of damage as a result of plant roots and they will note it.
I have worked as a Gardener for 24 years. My latest garden is a new build garden on heavy clay.
I expect most people don't deliberately watch their neighbours even though they can see them if they look.
From my upstairs back windows I can see into probably 5 or 6 gardens and I'm sure the windows are visible from more gardens than that (we're near a corner so the back of our house faces across the back gardens of the side street) but I couldn't tell you what the neighbours are up to in their gardens unless they're making a noise (can't switch off my ears!). I assume they don't take any notice of what I'm doing either, but if they do, good luck to them - I'm sure it's not very interesting watching me weeding, turning the compost, cutting the grass or just sitting in my chair reading.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
I understand the need for a sense of privacy, even though I know that nobody is watching me in my garden. About six houses upstairs look straight into my garden. I put in an arch to screen a little from overlooking and it really does make a difference to the sense of seclusion, even though it doesn't do that much in practical blocking terms. With high climbers over my fences and a climber-covered arch and tall pots to make a "private patio", it makes for a cosy feeling, especially when sitting down at the table (so that the vegetation seems higher) and I find that's enough. It's all illusion, really, but visceral stuff.
It's a little like having small ponds with mini fountains playing. It distracts from neighbour noise, aeroplanes and helicopters. The splashing doesn't really cover anything, but it helps and sets a mood and an intention. Visitors say "oh, it's an oasis" when it fact it's just a pot with a pump in it and some tall dahlias around.
If you go the obelisk route buy or make ones that stay a square shape all the way up baring the very top, not the ones that taper and go narrow at the tops like a cone or pyramid shape. They won't support enough plant at the top to make enough cover to hide you. The reason I suggested the globe ones are they are broader at the top so support and allow the climbers you choose to billow and fill a larger area high up. I found the "umbrella" ones. https://www.classic-garden-elements.co.uk/products/rose-umbrella-giverny/
Oops just seen the price on those ones. Faints. But I have seen ones that were home made with a big four corner obelisk with a large hanging basket attached to the top upside down for, I think it was, clematis.
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That border seems not particularly wide either so make sure you are going to give enough water to whatever you plant there.
Personally I find it strange all this privacy demand in the middle of a sense housing development...surely that ship dialed when you decided to live there. It's such a weird British obsession my European brain can't cope with 😉
Another consideration could be a row of multi stemmed small trees which will give you good height and spread once established. Rowans and Amelanchiers are possible options and widely available. Although you won't get a dense green screen like a hedge the view will be filtered during the growing season. You could use a mix of trees to avoid looking like an intended hedge and avoid the hedge height limit of 2m. I don't think you'll need more than 3 multi stemmed trees to screen off those windows and door. They will require regular pruning once they approach your required height though.
And it would make for easier maintenance of the fence and the climbers.
As it wold be a foot or two forward it would with the plants built up and billowing along the top makes for added height without too much space taken up or bulk.
Sorry if it sounds confusing I hope you understand.
Or a few good sturdy obelisks with climbers would also work . You can get ones with an "umbrella" style top and ones with a kind of globe too I think.
Also consider that although you feel very overlooked, the distance of those windows means they will only possibly see your heads or a little bit more from that distance and height?
If you look across from your upstairs windows what can you see on the ground or over the fences from your perspective?
Although you look up and see them standing in their long window/door, with balcony thing, they might only be able to see you a little bit.
Edited to add a link to the kind of obelisk I was thinking of.
I am not advertising, other blacksmiths and suppliers are available.
http://www.tatam-blacksmith.co.uk/iron-garden-2f13.htm
If one does die what next?
The Obelisk idea is a good one if you want instant impact and something to draw the eye away from the fence. Also you can take them with you when you move.
If your concerns relate to upstairs windows that is different.
I have just moved house and the surveyor for the old property' was on the ball' with the plants the slightest sign of damage as a result of plant roots and they will note it.
The reason I suggested the globe ones are they are broader at the top so support and allow the climbers you choose to billow and fill a larger area high up.
I found the "umbrella" ones.
https://www.classic-garden-elements.co.uk/products/rose-umbrella-giverny/
Oops just seen the price on those ones. Faints.
But I have seen ones that were home made with a big four corner obelisk with a large hanging basket attached to the top upside down for, I think it was, clematis.