Thanks -- it seems like I'm some sort of expert now.
UCL has a holistic admissions process and, all things considered, you've got a pretty strong application. People with BMAT scores like yours have received offers before (have a read through
this if you have time -- there's no real pattern, as you will notice).
There was a really good video that I watched which was an open day presentation by the admissions tutor for medicine, Dr. Brenda Cross, in which it was mentioned that UCL first screens all applications for minimum requirements, then uses AS grades, the personal statement, reference and, to a lesser extent, GCSE grades, to determine 'priority' for further consideration.
The BMAT scores are then used to determine suitability for interview, giving a final 'priority' to each applicant and going through them in descending order until all 700 interview slots have been exhausted. I wanted to provide a link to this but it seems that it's been taken down because I can't find it anywhere.
In that sense, you'll have a higher priority for further consideration than I would because of your high AS grades and UMS, despite my (slightly) higher BMAT score and comparable GCSEs. Don't worry too much; worrying won't affect the outcome at all so it's best not to do it and just check your emails regularly.