Also my MEP rating has expired. If I want to fly a G reg Seneca with my FAA PPL, how would I go about getting an FAA MEP rating and do I have to do an annual flight test to renew it?
You do the FAA ME flight test (there are no minimum hours) though you will have to do stuff like a Vmc demo as part of the test. I originally did my private ME rating from scratch in 10 hours inc test...but I guess if you have flown and done the G reg ME rating then it would be less for you. No annual test to renew. Your standard FAA BFR revalidates all ratings. You would need TSA and Visa to do it though.
You can get your 61.75 certificate in the UK but it will cost you £3-400. It is free in the USA.