I did my A&P back in 1991, so things may have changed, but i don't think they have.
You can take the exams seperately, there is/was the airframe, powerplant and general paper followed by the oral/practical test. I doubt you will be able to have a "restricted" A&P with just an airframe or powerplant rating.
As for other certs - I think getting a job with a company that operates a lot of N reg aircraft will be the hardest part, never mind additional certs.
You could give the feds a call at their West Drayton office on 0208 754 8819.
They are friendly enough. Roberto Gonzalez is your man, or he was last year.