It makes sense if they would like to throw the buyer off the scent and suggest it is a UK-centric service.

But a bit non-sequitur with the rest of the email - American English spelling and while Bill is quite common in the UK, Jarrett is an unusual name there.

They already knew we were American, we weren't hiding the fact.

Agreed. £10,0000 would have been the correct way. Or 10,000 GBP.