I find Dash4it pretty good, and they are very generous with their discount codes as well.
They have just about every map you would require, for the entire Uk and the whole of Europe, but its their prices i particularly like.
My last purchase from them, were two of the latest O/S maps covering most of Mid Wales, for a tenner which included delivery.
There are not many reputable companies selling brand new inclusive of delivery O/S maps for a crisp £5, and they cant be selling them at a loss, so just goes to show what mark up other companies are making.