I did masses of research before I bought my mattress and what I found out was if in doubt, always go for a turnable pocket sprung - as most experts seem to agree they are far the most supportive. Included memory foam/pillow top/latex top mattresses are kind of gimmicks - as being able to turn your mattress makes it last far longer (manufacturers love non-turnable mattresses as it saves them money, as they only need to put expensive fillings on the one side and then sell it to you as if it is a good thing! )
If you want memory foam, latex or pillow tops they suggest buying a separate topper, so you can still flip and turn the mattress underneath. Also don't be fooled by paying more for descriptions of cashmere or silk etc - apparently the mattress will only have a miniscule amount as it's too expensive for manufacturers. Though supposedly often the more you pay, the more breathable natural fibres like cotton and wool will be included - although the main bulk of any mattress wadding will still be polyester.
They say to aim for 1000 -2000 springs for a king size, depending on how much you want to spend. Less than 1000 will not be supportive enough, more than 2000 is another gimmick to get you to pay more as they can't fit them all in 1 layer, so again risks compromising support.
I'd try to buy from local smaller sellers if you can as they sometimes have lesser known brands that are better value. Most UK mattresses are apparently made from similar materials in West Yorkshire factories, so you can be just paying more for the "name" for big brands.
While it's really better to try mattresses in person, I realise that isn't possible at the moment so if you aren't sure what firmness you like, I'd suggest sticking with Medium Firm as it's by far the most popular (although what Medium Firmness feels like seems to vary hugely between manufacturers unfortunately). You can soften the feel of a mattress by adding a topper or make it feel slightly firmer by adding plywood underneath the mattress (or sometimes even adding an electric blanket on top makes it feel a bit firmer).
Make sure you have a supportive base to also help a mattress last longer too - the best is a sprung edge divan (gives most support, slightly softens the feel), then a cheaper platform top (firmer feel), then finally bed slats (but make sure you only have very small gaps between the slats - add extra if necessary or you can put plywood across to help support the mattress properly).
Sorry about the essay but it helped me so I hope it helps you too!