Caveat: Although I am trying to be impartial in advice I do work for an FTO (BCFT).
The most important thing to do in choosing a school is to visit all those you think are possible, and judge how the "feel" is. Different people look for different things, and most students who post have attended only one groundschool, so cannot really compare them. Thus Oxford look good because they have a larger number of students, some of whom post here. The majority of feedback is positive for all the schools that have stayed in business.
A visit will also give you some idea of how organised the admin is (as FFF says this is important, especially for distance learning). You get to meet the instructors, and get a look at notes etc (there are bad notes out there, though not those used by your shortlist of schools).
We don't have approval for distance learning yet so I can be impartial in recommendation, and I agree with FFF that Bristol are better than Oxford (I have helped students of both schools with private tuition), although I have nothing against the latter's DL course.