best mattress for us on a budget of £600

Hi John and Ryan,
I would like to find out the best mattress for both me and my husband. We are 12 stones (5ft 3) and 15 stone (5ft 9)
This will be our first "proper" mattress as in the past we have opted for budget mattresses and are now suffering from back problems
Our budget is £600. What would you recommend?