A most interesting questions, @Ceramiq. It depends entirely on the personal qualities of the applicant.
McGill has a (much) stronger research reputation and is likely a more difficult programme. The pedagogy is North American, meaning that there are likely to be weekly or biweekly assessed assignments in most Maths modules as well as midterm exams. I don’t know about McGill specifically, but in NA students typically take about four courses per term with at least one external to their primary field of study.
I supervised the academic side of Study Abroad for my School for a while. Even though we are in the tier just below COWI our students who are selected for McGill, which is one of the most competitive destinations, find that it is a big step up. However, they mostly adjust, love it, and do very well. (My subject is Maths intensive and our students take Maths classes at McGill)
Of course McGill is in Montreal, a truly international city. One need not be bilingual to make Montreal home, but most who live there are.
Roughly speaking, my impression is that North American universities offer more academic support than British universities and students are expected to use it as needed. OTOH there is typically less identification with a School. In the first two years there may be many General Education requirements. You won’t be seeing the same faces in most of your modules, the Subject Building is less likely to have a good food hub IME, so just going about life gives fewer opportunities to fall into friendships. You may have to join sports, societies, do hobbies, etc to make friends. Luckily North Americans are very friendly, and Montreal is a wonderful city.
The lesser identification with a School may or may not have ramifications for students with SEN. It is worth finding out how their special circumstances (which I would always encourage them to use) are administered. Is it convenient for the student?
On the whole Bath has very happy STEM undergraduates, and this includes the mathematicians. It is known for excellent maths teaching. The research may not be as intensive as McGill’s, but it is vibrant - DH thinks highly of some people there and more importantly, there is more than enough to give SoM UGs the connection to current research that does benefit the brightest.
Bath also has an outstanding reputation for extracurricular activities, particularly sports. The campus just has a great vibe! The students look happy. The reputation for working with SEN students is very good.
The city is small but very beautiful; Bristol is down the road and London is a reasonable train ride.
DH and I don’t know much about the quality of UG teaching at McGill. The candidate could probably find a relevant thread on reddit or similar. However the UK really is a leader in teaching methods so as excellent as McGill is, I would be surprised if the quality of the lectures and teaching materials is higher. OTOH the more intensive North American pedagogy may help the material stick better,
Which learning method suits the applicant best? Which living environment? For a very strong applicant with a good work ethic McGill has the edge; otherwise probably Bath. What a lovely dilemma