Which mattress for back pain, 14 st woman 5'5" and 14 st 6'1" man

I would like a double mattress firm-ish support but soft feel for top (as long as support comes through) lower and middle back pain for a side sleeping 5'5" woman 13st 10lbs my partner is approximately same weight but less issues with pain he is also a side sleeper
Our budget is about £800