All I will say is, don't let the glamorous lure of London drag you into going to QM. Also, don't listen to arguments about being "around" law. London is a lonely, impersonal place and you'll get just as much chance at extra-curricular activities at Exeter because it is a great law school.
Biggest thing to consider for me would be overall reputation. Exeter is far better than QM, so that in the event you don't want to go into law after you graduate, an Exeter degree will be held in better regard to a QM degree. Whereas QM's law school is (I believe) one of QM's highest ranking departments, leaving law to be one of the only career options where you degree will be respected.
Overall, though, it's all about where you think you will be more comfortable and in turn, get better grades. You can't go wrong with either decision you make.