Not used these personally, but they've got good feedback from buyers over at ebuyer...
http://www.ebuyer.com/UK/product/92986
Logitech OEM Speaker S200 2.1 Stereo Black Silver
£15.48 Inc. VAT
http://www.ebuyer.com/UK/product/74746
Labtec Pulse 485 28w RMS 2.1 Subwoofer - Retail Box
£19.25 Inc. VAT
Seem to be a decent price.