Bought this a few months ago:

John Lewis & Partners Memory Collection Compact Memory Foam Roll-up Mattress, Medium, Single
£175.00 ... lsrc=aw.ds

It cost me £175. 7yr guarantee, although they do say it's ''only for guest use''. So, I changed my surname to ''guest''.

Been lying/sleeping on it for about 12hr a day for a few months. I weigh about 53kg, but am now very skinny on my spine, bum and hips. Prone to the beginnings of pressure sores, which never fully heal (neuro auto immune 18yrs + recently had more weight/fat loss).

So far, so good. Much more comfortable than old box spring thing. I don't have to turn frequently and the memory foam is warm. Only problems have been when I added a plastic undersheet to protect the mattress, it became too cold to sleep on.
Also it is about an inch shorter than my old mattress. There is a clear gap in my bed where it is ''missing'' that inch. Bit weird, and I do find myself having to position myself to avoid toes slipping off the edge a bit (despite being just 5 ft 7).

However, given the price, overall I am very happy with it. Would recommend for those on a budget, provided you're short and lightweight.