I think that anything big enough to screen those windows is going to stop almost all light into your own upstairs windows, I would go for something over the patio as K67 suggests, perhaps a pergola with something whispy climbing up it.
My upstairs windows are just the bathroom but I have been thinking about a pergola. Actually when I sit the other side of the garden I don't think it would need to be as tall so a pergola positioned well might be my answer. I'll have to start thinking and measuring.
Other than that I wondered about a dwarf buddleia as I already have one in the border that was bought fairly small but in four years is now just above the fence. If it was in a raised bed that would make it higher!
It was K's idea, but I have recommended them before. A pergola in the right place can be a great solution to being overlooked, as you say. Sails are widely available from online garden shops - Harrod, Primrose, Amazon, Wayfair. They start from about £20 and come in all sorts of shapes and scales. They from like parasol-types to covering half the garden. Some are easy to take down when you fancy.
If you go for bamboo a fargesia variety is clump forming so no worries about it escaping. I paid £30 for a 5ft plant and it will grow to around 3 or 4 metres. A raised bed would be best especially if you can take up the flags underneath and dig up whatever is under them, ideally you need around 900cm of good soil. Photonias grow fast and are evergreen but you would need to clip them to keep them neat. Those silver birches look amazing but at what cost? The tubs alone must cost a fortune!
Hopefully you mean 90cm, but even that is 3 feet in old money and would be a lot of digging. I have a Fargesia Nitida which has been growing for about 15 years and is still only just over 6 feet tall. As it grows it tends to create a weeping form so although the bamboos may be 8 feet long or more they aren't just going straight up.
Sorry! I work better in feet! There are more upright versions of fargesia. The one I have is fargesia robusta pingwu. I was assuming that there would be a raised bed or perhaps a large deep container with the bottom cut out and it sunk into the ground a bit to give stability in the wind. It depends if there is any soil under the slabs.
My neighbours have a beautiful large sail but they take it down in the Autumn and put it away until late Spring when they give it a good hose down ready for Summer. I believe this is to protect them from high winds and snow, as this could rip and damage them.
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Other than that I wondered about a dwarf buddleia as I already have one in the border that was bought fairly small but in four years is now just above the fence. If it was in a raised bed that would make it higher!