Since the 1 May of this year the only places you can do the written bits outside the US are those schools which are Part 141 approved.
That is currently only FlightSafety at Le Bourget and in the next week or so American Flight Training at Norwich contact details are on their
website.
The latter has examiners coming over regularly for people taking flight tests.
You will at some stage as 2Ds says have to fly with an FAA instructor to get the sign off and you will need 3 hours of instruction within 60 days of doing the test.
Getting the M1 Visa is a minor pain if you follow the guidance offered by the US embassy and in one of the sticky threads here.
What works best for you depends on a number of variables.
Hope this helps, if you want to know more feel free to ask!
FD