TJ hughes have a sale on memory foam matresses at really low prices. The link above gives a matress from Sleepbetter where a double is £149.99 reduced from £499 and a king for £199.99 reduced from £599. Both having a 5 year warranty.
There are two other brands Silent night and Sealy both slightly more expensive with Silentnights King £249.99 reduced from £609.99
Bargain in my opinion.
Not sure on end date??