I sent off for one from the Daily Mail and it is fantastically comfy. It cost around 75 quid and is about 3 inches thick. You can buy really expensive whole mattresses but they are essentially just 12 inches of normal foam with 2 or 3 inches of memory foam stuck on top.
I've never slept better (once the chemical smell wore off!)
While agreeing entirely as to comfort with dimli, I'm just a little concerned with the one we bought when warm weather finally arrives. While last night's -18F here in the inter-mountain western U.S., made it enjoyable for it's warmth, it may turn out to be too warm in summer... we'll see...