Just remember that Ikea beds have different dimensions to every other bed in the UK. Which is Ok if you buy both bed and mattress there. My current IKEA bed has a nice exposed bit round the edge where my new mattress doesn't quite cover it!
Do you need a whole new bed, or just a new mattress?
Loads on ebay as well, eg
pine double bed frame | eBay
4ft 6 double replica Victorian dark grey / pewter metal bed frame / bedstead. | eBay
New Pine Double 4ft 6 Bed Wooden Frame | eBay