I got into Goldsmiths as a mature student (I was 23) with a BTEC Diploma (Triple Dist) and an AS Level (B) I had no GCSE's or A levels. If you look on the UCAS points board, having a diploma with good results is worth heaps of points.
I found my interview fine, I didn't think I wowed them or anything and found it difficult to answer many questions, but the interviewer in question was nice enough.
As for it being "pretentious" well I think it is what you make of it. I found it hard to fit in as I was from a different walk of life to most of my fellow students and yes most of them are middle class, but you get on with it and just don't take any crap from anyone!