-
We went with these for our first floor because we also wanted flat fronted and are happy with them so far:
https://www.heatandplumb.com/acatalog/maxheat-obla-flat-panel-radiator-ks508fThe aim was basically to get them to disappear into the wall behind as much as possible.
Very similar to the ones @dbr linked to but it's hard to know what the quality is like until they actually arrive and for some reason the photos of radiators online are always crap. These ones come in lots of different sizes which was a consideration as we'll be replacing them in the whole house (they don't just come in 500mm width, there are separate listings for each width).
My plumber confirmed that single convector is fine for upstairs rooms.
If you're going to DIY I can give you tips!
We are going to move and replace an old radiator in our bedroom. Does anyone have any recommendations for a slim (projection from wall) and ideally flat fronted radiator? Online calculator says it should be 2000ish btu but we (I) don't like a hot bedroom so I'm willing to balance output and size.