Help choosing a mattress

Hi, after extensive conversations a few years ago I ordered the "Fusion zero - Double - Firm Tension" and it has been pretty much perfect, so thank you for that! We're now looking at getting a bigger bed so are wanting to upgrade to a super kingsize, can you recommend which mattress in your current range is closest to the one we currently have? A little background info if it helps, I'm 26 years old and suffer with rheumatoid arthritis, I weigh around 95kg and am 6ft tall, my partner weighs around 85kg and is 5ft 9". I don't think we have specific sleeping positions as I tend to be all over the place, if I had to guess I'd say I'm more of a side sleeper. Our budget is around £1500 but I'd prefer to also get a base and headboard in with that cost too. Finally, our current bed was a return from someone else I believe so was slightly cheaper, I'm fine with doing the same again if it means getting the correct bed. Thanks for your help!