Superbeast, have you heard of bigwords.com? I don't know how useful it would be to you in Britain, or if there's a British version, but it really does obviate all other bookstores. It's not a bookstore itself, but a searcher of all other bookstores.
Compile your ISBN#'s. You have a long list, I'd put it in an excel sheet. This way every time you use Bigwords.com, you can just copy and paste.
Bigwords will search every item in your list, then give you a best single store price for all items (it factors in additional shipping charges for you), or give you a best individual price for each item. You can also select new books only, new & used in good condition, or just plain old used.
Truly, it's mind-boggling.
