Terrible customer service. Failure to respond to emails and fobbed me off with outdated, irrelevant automated replies. Supplied a tracking number that they claimed repeatedly was for Royal Mail, turned out to be Hermes who refuse to deliver to me so at that point I knew the book was never going to arrive. Over a month after my order date, and three requests for a refund later and having to involve Abe Books investigation team (who were amazing) I finally got a full refund.
No idea what kind of scam they're running but I suspect they take your money and hope that, if your book doesn't arrive, that you'll give up chasing a refund if they ignore you for long enough.
Absolutely nothing great about Great Book Prices UK. Just a great big con.