Most suitable mattress for me and my wife

Hi, Your website has very useful information. Can you advice the most suitable mattress for me and my wife -
1) My weight is 12 stones 10 lbs (height 5'7") and wife is 7 stones 2 lbs (height 5'2").
2) We both prefer somewhat firm feel to our mattress.
3) We both are side sleepers but often switching to back sleeping.
4) We do not want the mattress to be warm - as we frequently tend to get warm
5) We want to go in for a superking sized mattress - we have a ottoman style divan bed with sprung slatted base
6) While we are willing to spend upto £1500 or thereabouts for have the most suitable option - of course will be happy if we can find the right solution in less.
In addition to suggesting the most suitable product from your own range, can you also kindly let me know which is the closest equivalent option on John Lewis website for the above requirements. We have specially looked at JL own brand (John Lewis Natural Collection 5000 Linen (http://www.johnlewis.com/john-lewis-natural-collection-5000-linen-pocket-spring-mattress-super-king-size/p2429064) or Vi-spring Dratmoor (http://www.johnlewis.com/vispring-dartmoor-superb-mattress-super-king-size/p1577515).