I'm in a similar boat as you: bad back, currently looking for a new mattress. I can't really offer any advice or recommendation per se because the more I've looked into it the more confused I've become and have yet to take the plunge.
However it might be worth flagging some of the issues that I've come across on the journey of scouring the internet for info & reviews.
1) As per Barnacle's advice the general thinking on memory foam casper, emma, eve, simba et al have very mixed reviews. Seem good at first, but more concerning later. I think the jury is out regarding longevity.
And the 100 day trial seems to have come in for lots of criticism. Worth reading the small print as apparently some require you to repack in the same packaging which it came in. Considering they are vacuum packed this seems to be sailing close to the wind as far as unfair contract terms goes.
2) There's lots of newspaper comparison reviews on the internet - independent, guardian etc. all with articles such as top 10 for 2018. On the back of these I looked into the premier inn and other luxury hotel chain matresses which come highly rated - on the thought process that commercial use must be better quality. It's probably true to an extent, but again there are mixed reviews.
3) finally I came across the following website, which seems to be far more honest that some of the big brands and are prepared to reveal the actual construction and materials they use - unlike all of the big brand manufacturers. Plus they seem competitively priced. Worth taking a look as you really need to choose depending on how much you weigh etc. https://johnryanbydesign.co.uk/product/origins-pocket-1500/
(you need to scroll down as the techy advice bit is down the webpage.
4) last bit of unsolicited advice (sorry, can't help myself) is get a copy of the 'back mechanic' off amazon. There are 3-4 key exercises to do, but more importantly a whole load more to never ever do again - like sit ups.
Let me know which one you go for as it might persuade me to get on and decide.