I did a lot of research into our mattress mainly because Dh and I are very different body weights, he is 6'3" and used to play rugby so he is a solidly built bloke.
We knew that we would need a "firm" mattress to support him but I didn't want to feel like I was sleeping on a plank of wood. I read up a lot on lots of different websites and buying guides.
The best place I can point you to website wise is here warning it plays a video the second you open that link, he cuts open two £200 mattresses and reveals that the branded one is in fact a cage mattress (cheapest mattress spring and not good quality) despite its fancy name.
We went to a local factory bed place, ie they make the mattress on site and we could dictate what we wanted in the mattress in terms of content and finish. We also chose a divan base where we could choose the colour of the fabric wrapping round the divan all the way down to the castors on the bottom.
Now I know you are thinking that this cost a fortune, well it didn't in comparison to what we could get on the high street. We actually have 2x3ft bases and zip and link mattresses. So our bed is 6ft wide with two 3ft mattresses on it. We put a quilted mattress topper on so that you cannot feel the join.
It cost £1000 for the whole thing, but we have a lot of natural products in it to keep DH cool, went for pocket sprung and a high spring count and we now refer to our bed as "the cloud".
Plus our mattress is two sided meaning it should last twice as long as we can flip it. We live in West Yorkshire but there are factory bed places all over where they make the mattress not just buy it from somewhere else. I would talk to them, they can surely custom a mattress to your budget.