I just asked
http://www.fxcompared.com/ for quotes for 20,000 GBP to USD. Transferwise was one of three identical best results (32,784 USD). XETrade offered me 32,711 USD just a few seconds later.
So Transferwise appears to be worth considering. I can't find a way to get an Amex quote without signing up (or finding out how many MR points I'd earn). Anyone?