I was going to suggest that, but I think they might only be valid ex-USA, and not ex-LON. Is that correct?
Edit: I just checked the T&Cs and did not find that...but I seem to recall reading such...so I'm not sure.
Cheers.
I know they are valid for int'l travel but it is a good point regarding departure.
Note to the OP: if you pursue this route, determine whether or not the discount can be applied to a departure from the UK.
If not, I wouldn't worry too much. W/the current dollar/GBP exchange rate, you'll find just about everything in the states a bargain these days.