Barts does take the UKCAT into serious consideration and a score below 2200 probably won't get you an interview, but they can exercise judgment in the 2200-2400 range based on your personal statement.
As far as your options go, its all about the fact that they're three quite radically different schools. UCL is all but completely merged now, so you'll spend a lot of time (At least preclin) with people from other courses and generally have the Goodge Street student area life. Barts is semi-autonomous, mainly charactised by an extremely active student body with a talent for punching about its weight (Know another college where students regularly raise £3,500 each in a week for charity?). St.Georges is similar to Barts, but separated from central London (Both good and bad) and acts more as a college of healthcare with nursing and associated professional courses as well as medicine.
Personally I'd say Barts because I have little griffin and shields about half of my clothing. But then I would say that. I don't know who you are or what you would prefer, so just apply where you think you'll have the best time.