I am looking for a new mattress, me back pain, wife shoulder pain

My wife is 65kg and 5 ft 10. I am 78kg and 5 ft 11. We are active and athletic. Our current mattress is a Hamilton & James Luxury handmade 2000 springs/" title="pocket spring">pocket spring. We have a wooden frame base with sprung slats. We bought it 5yrs ago. It was expensive and has never been right for either of us. Archers wouldn't take it back even though we immediately asked to change it. My wife has always found it too firm and finally got a mattress topper which i didn't want, but accepted. I found the mattress a bit firm too but didn't like the topper and over the last 18 months began to developed lower back pain which is inflammation of my L4 & L5 joints. It is constant chronic issue now which is effecting my ability to do sport and is proving very difficult to rehab. According to my physio it is likely to be at least partly related to the mattress. My wife also has shoulder pain which may also be partly related to the mattress. My wife is a side sleeper. I am a side or front sleeper. I think i move onto my front because side sleeping is uncomfortable, which i am beginning to suspect (following research on your site) is to do with an overly firm mattress and pressure on my shoulder. I think that the topper aggravated my lower back because it didn't offer support but pushed my back into a more extreme extension when i slept on my front. So, with that all said, what would you recommend as the best mattress in terms of firmness and comfort? Happy to go above £1500 or even £2000 if i have confidence that it will help resolve our issues. Many thanks, Craig