LOL Pete! I have to put my hand up and admit that, as the owner of several Martins, I have a vested interest in knowing these things. But I've never taken any of my guitars to the UK AUthorised Service Centre, I use trusted and respected luthiers/repair guys local to me (i.e. within a 90 minute drive).

The Martin Lifetime Warranty only applies to North America-purchased guitars, those bought in the UK have a one-year warranty only, so it matters not a jot who does the work (just take care to choose someone capable!).

Usual disclaimers.......IMHO, YMMV, etc.