Hope you have a good trip to the Big Smoke!

If it's a modern-ish American modem and the outer British pair connect to the inner U.S. pair then it should work whichever way round it's wired.

I'm sure you realize that strictly speaking it's illegal to connect anything to the lines which doesn't have BABT approval....... [Linked Image]

(Paul exits quickly, muttering something about having to edit out references to American 500-type phones in his house. [Linked Image])