Hi ademuzzy,
Unfortunately … I don’t have any particular knowledge about the UK market so I won’t be able to help in terms of specific retailers or manufacturers I’m aware of there but the brand name of a mattress isn’t particularly important anyway because outside of how suitable a mattress is for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences), a mattress is only as good as it’s construction and the type and quality of the materials inside it (see this article and the quality/durability guidelines it links to) regardless of the name of the manufacturer and the mattress materials that are used in the UK would generally be the same as the materials in North America (see post #5 here about “brand shopping”).
The only caveat is that in North America the foam density guidelines for polyfoam and memory foam that I would normally suggest are expressed in lbs/sq ft while in Europe they are often expressed in kg/sq meter so you can use the online density converter here to “translate” them.
I would tend to look for smaller local and regional manufacturers/retailers or online manufacturers/retailers that are completely transparent about the type and quality of all the materials that they use in their mattresses so you can make more meaningful comparisons between mattresses and identify any lower quality materials that could be a weak link in any mattress you are considering.
Forum searches on .uk and on UK market (you can just click the links) will also bring up some posts, comments, and replies to other members from the UK that have posted here (with a few “extra” that may include the search terms that aren’t from the UK) that may be helpful as well.
Phoenix