It isn't compulsory in many or most schools. But some schools do have set subjects that they insist all children take. R.S is a common one because, like P.E, it is compulsory for the school to teach it beyond KS3 so some schools reason that the students may as well get a qualification in it. Other schools have option blocks which mean that a student taking drama can't also take music or a student taking art cannot also do photography. It depends on the size of the school and their timetabling.
As for university - if DD gets level 9's across the board, no university is going to be at all put off by a BTEC in P.E. If anything, it will show she has breadth.