I phoned cambridge/ucl/oxford about this and asked if it would make a difference in the grades you would need to get for medicine. All said no difference. Personally I think it is not better to sit as an external candidate cos the grades are harder to get.
Sir,
I am an A-level Student who's in grade 11. I achieved 8 A*s and 1 A in 1 session (June 2009). This year I'am taking, as an external/private student, Maths, Physics, and Chemistry A-levels (AS and A2), ALL IN 1 YEAR! I know that seems a bit crazy, but truly it's not that hard. I believe that it all depends on the teacher. Anyway, thanks a lot for your advice..
(waiting for my AS results in march!! Need your prayers Sir!!)