How deep are the raised beds? I would probably use almost entirely screened topsoil, and then dig in a soil conditioner such a well rotted manure. Pure multipurpose compost can degrade and sink over time, becoming compacted and lacking aeration.
"What is hateful to you, do not do to your neighbour".
You could consider a specialist supplier like Quickcrop. I’ve used them here in Ireland but they also have a UK operation. They sell bulk bags of “vegetable soil mix” which is a mix of screened topsoil, compost and sand in the ideal proportions for growing veg. I found it to be of excellent quality and didn’t need anything else added. Link to the product in question here (Mods, please remove link if not allowed): https://www.quickcrop.co.uk/product/bulk-bag-of-vegetable-soil-mix-england-and-wales-delivery.html
I agree with the topsoil idea. Combined with some well rotted manure it’ll hold moisture and nutrients much better than bagged compost. Over the years raised bed soil starts to resemble bagged compost anyway because of all the organic matter we lovingly pile into it. It becomes very open and free draining and mine became a nightmare to keep watered. I mix in topsoil to maintain the level now and I think I’ve had better results.
That's exactly why I mentioned location and climate as factors @DaveGreig. The last thing I need in my raised beds is a mix that holds onto moisture. It's the reason for having the beds to start with! The mix I have is ideal for all sorts of plants and is largely that ideal that's often mentioned - moist but free draining. Turf/soil in the bottom, then generally compost was added, but home made as well as spent stuff from annuals etc. It does settle, but that's normal, and it's why regular additions are used, and also care is taken when planting anything, so that it isn't too low to start with. I'd agree that manure is ideal if it's mostly for veg growing, although even then, some won't like terribly rich soil, so as always, it depends on the plants being grown.
It's a place where beautiful isn't enough of a word....
I live in west central Scotland - not where that photo is...
I do not want to discourage you far from it but if you were to purchase individual bags of peat free compost it would cost you £400 to fill those raised beds. Filling raised beds is always a problem that the raised bed brigade do not always realise until it comes to filling them. My wife has 2 large ones and one that runs the full width at the bottom of the garden. When I built the 2 large ones we planned it like a military operation. Because were we live we have 6 trees we collated some of the leaves from the previous year rather than taking them to the allotment to dig in and then in an old Honda CRV transported bag after bag of farmyard manure from the allotment and then pinched bag after bag of topsoil from the allotment ( don't tell the council ). As other people have said you will constantly have to top up and the raised beds even thick sleepers like we have will at some point need to be replaced. They will need lining otherwise they will rot before your very eyes. I used Asda banners where I once worked and I hope to get 10 years out of them. Because we still have the trees I have to remove all the tree roots each year I bought a Canterbury Fork to help do this but at my age it is still hard work
I watched a chap on youtube although as he was in America I should say Guy, he mentioned filling the base with wood cuttings part rotten is the best as the cuttings hold moisture.
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I'd agree that manure is ideal if it's mostly for veg growing, although even then, some won't like terribly rich soil, so as always, it depends on the plants being grown.
I live in west central Scotland - not where that photo is...
When I built the 2 large ones we planned it like a military operation. Because were we live we have 6 trees we collated some of the leaves from the previous year rather than taking them to the allotment to dig in and then in an old Honda CRV transported bag after bag of farmyard manure from the allotment and then pinched bag after bag of topsoil from the allotment ( don't tell the council ). As other people have said you will constantly have to top up and the raised beds even thick sleepers like we have will at some point need to be replaced. They will need lining otherwise they will rot before your very eyes. I used Asda banners where I once worked and I hope to get 10 years out of them. Because we still have the trees I have to remove all the tree roots each year I bought a Canterbury Fork to help do this but at my age it is still hard work