10 cm mattress topper or base to ease John Lewis firm mattress

We wavered between firm and medium when buying John Lewis own brand mattress (£799 for 150cm width) thinking firm would ease slightly and medium was not firm enough. After 2 years it hasn't. Tried a couple of toppers and I'm wondering if a sprung bed base laid on the wooden base below the mattress would be an answer. Or even a good quality slatted base perhaps with a 7-10 cm frame rather than the usual 5 cms to give adequate clearance for the springs bending