Ikea have reduced the price of their Hovag mattress:
Standard single £135 (was £150)
Standard double £180 (was £200)
Standard king £230 (was £250)
This mattress was rated very highly by Which.co.uk (best buy) and seems to be very comfortable!
It has 1200 pocket springs with a layer of foam (not memory foam) on top.
The medium firm one is actually quite soft, I have gone for the firm which feels just right. Link goes to standard double firm mattress.