Mattress selection that is as natural as possible?

Mattress

We are looking to replace a mattress and our requirements are:
Super king
Firm/ med support
Probably a topper to provide a luxurious sinking feeling
As natural as possible
No heat retention
Husband: 14-15st; wife, 11st

As little turning as possible due to the size of the mattress.

Budget £1k - £2k

And pillows needed too.

We would like to buy from you so your advice will be much appreciated.
Kind regards

Lorraine