I've worked with people from both schools and they were all of an extremely high caliber. As such, I don't think that there is a difference in the quality of the education. I also agree with other posters that both schools should give you a great chance to get into MBB. I would therefore recommend looking at other factors:
(1) Visa: Please keep in mind that Brexit could cause troubles. I'm currently on a visa in the US and visa issues are a bigger problem than one would think...
(2) Cost of living: London will be more expensive than St Gallen and I would recommend estimating the total difference over the span of the entire MBA. This is doubly true if you have to go into debt.
(3) CV Optimization: Which school will round out your entire CV better? For example, should you be from the UK, it might make sense to go to St. Gallen.
(4) Industry foci of MBB offices: Taking into account which industries you would want to work with when in consulting can help you decide. For example, I would expect a large amount of finance/banking projects at London MBB offices.
Lastly, you have two great offers - leverage them! GMATClub has several threads on using multiple offers to gain financial aid from the universities and I would suggest at least trying to do so.