Futurlec - Good supplier, very cheap SMD to DIP converter's
RS Components - Not always the cheapest, but you can collect from a trade counter. Shipping is also quite expensive if not a large order.
Farnell - Larger selection than RS and free shipping, however there is a minimum order to qualify.
Digikey - Even larger selection than Farnell and free shipping on orders over £50. However, if you don't meet the £50 threshold the cheapest shipping option is £12.