Mattress and base combination 18st and 11 st

Please advise how best to accommodate both our weights of 18 st and 11 st. We are restricted to a king size as maximum.
Our current bed does not give sufficient separate support and does not have a spring base. It is described as firm and has an integral mattress topper.
Could we purchase a spring base only to see whether the support improved sufficiently?
If we were to purchase a base and mattress together, please advise:
The benefits of a spring base;
The reason for having a mixed medium/firm pocket spring single mattress or having a zipped together medium and firm mattress.
We would prefer a single mattress or the sense of only being on one mattress. We do not want an integral mattress topper. What is the effect of placing a topper on the zipped together mattresses?
If you think you can offer us a solution, what best meets our needs? We have been looking at Vispring sold by John Lewis and the John Lewis range but think you offer greater value for money.