I would suggest that you stay off studentroom and make sure that you get your 4As before worrying about entry requirements to UCL.
You can resit AS modules in the upper 6th. Universities are interested in you being able to get your grades within two years, not necessarily when you first take the exam (i.e.- you can resit your AS modules without hindrance).
UCL do use the BMAT results in their selection for interview- if your predicted grades match their entry requirements (AAA for 2010) and you achieve 5, 5, 9 in the three sections respectivly, you are guaranteed an interview (we were for 2009 entry, I am not sure whether this still applies).
I hope this answers your question.