I had an interview at Imperial and by the looks of it, they interview in BMAT order. I think Imperial place quite an emphasis on the BMAT because at my interview, there was a person with no A*s at GCSE.
As mentioned before, Oxford decide who to interview with a combined BMAT/GCSE score and UCL automatically interview those with 5,5,9 so this suggests this is a good score.
In my opinion, Cambridge looks mainly at your UMS scores and your BMAT; there just happens to be a correlation with high GCSE scores.
However, once you receive an interview at any of the aforementioned unis (bar Cambridge who invite you to interview before the BMAT scores are released), it really is the interview that gets you an offer or otherwise.