Egdi,
Oxford medic's advice is good.
Our experience is that even doctors have their prejudices, and so can give contradictory advice. One London consultant claimed that the Imperial F1s she had were streets ahead of the Cambridge ones, especially when it came to communicating with patients and dealing with NHS bureaucracy. Whereas a Cambridge educated friend was raving about how much science background and understanding their Cambridge educated F1 had, and is rather sniffy about DDs course. DD says that some of the very best on her course came through the foundation programme, not least because a good Bristolian accent does no harm when communicating with patients.
It is about fit.
UCL and Imperial (BMAT) are different mainly because they are in London and because they do not offer the same strict divide between preclinical and clinical. However they also offer six year courses, whilst KCL and QMUL (UKCAT) allow intercalation. DD, who ran across a bunch of Imperial students at a conference (conferences are a good way to extend knowledge in an area of interest and again it does not matter which medical school you are at) said their course seemed a lot more science heavy. Again like Oxbridge, there is a lot of research going on: www.bidwells.co.uk/faqs/what-is-the-golden-triangle-in-the-uk/ (There are some phenomenal new science research centres in London, but again lots of intercalation options there.)
Like Oxbridge they can select/attract some of the more academic of the cohort.
DD is at Bristol, where it is unusual to have more than three A levels. This would not be the case had she gone to Oxbridge/London. Strangely Bristol used to have a reputation for rejecting some very academic applicants who then sailed into Oxbridge. The course offers lots of early clinical exposure which she loves. She is far happier doing than writing. Bristol also has a varied catchment ranging from proper inner-city to very rural with market towns and sea-side towns, with an aging population, thrown in, plus a couple of very good teaching hospitals/regional centres so may offer a better all round experience than, say London.
There is a huge range of roles in medicine. DDs dyslexia, practical skills and ability to retain knowledge means she is more likely to find herself in a high-paced "think on your feet" job than one which requires a lot of report writing. Her course plus a targeted intercalation should get her where she wants to go.
Her closest friend has always wanted to be a GP. Though she is eligible, she does not want to intercalate, not least that sixth year is expensive and means that you are a further year away from actually earning. The course is perfect for her, but again had she gone to Oxford there is nothing that would have prevented her from becoming a GP.
My advice would be to go to some convenient open days to get a feel for what different courses offer. You won't know where to apply to until you have UKCAT and BMAT (if taken early) results, but it is useful to know what appeals. And then not to worry too much. If you know where you want to get to, as Oxfordmedic suggests, and you have the skills and application, you probably can get there from any medical school.
In short - don't worry too much.