I have found this that may be helpful.
Over 22% of gradutaes that obtained pupilage, they attended a For clarity, The Russell Group university, comprised of 20 major research-intensive universities, namely: Birmingham, Bristol, Cambridge, Cardiff, Edinburgh, Glasgow, Imperial College London, King’s College London, Leeds, Liverpool, LSE, Manchester, Newcastle, Nottingham, Oxford, Queen’s College Belfast, Sheffield, Southampton, University College London and Warwick
24% were from Oxford and Cambridge
MOST ACCEPTED HAD A
Upper Second = 243 (52.8%)
MOST ACCEPTED WERE AGED
216 pupils aged 25-34 (47%)
So, in all, if you attended UCL or Kings for example, achieved at least a upper second class and were aged about 27 at the time of applying for pupilage, you stand the best chance.
That leaves room of 11% (approx) to people from other Universities like BPP and London Met, to gain a place. Out of the 400, 11% is just 40 odd places. Considering you get an excellent result and have a lot to put on your CV with relevance, I think the chance is there. To improve your chance, go to a better uni but if you just want A chance, you can study wherever. Although, obviously if someone had terrible teaching standards I would not consider it. The profiles of the teachers at Croydon are not bad to be fair, it seems that UCL is better "connected" and as explained to me by a pupil supervisor, the Bar Council invest in Law departments at the BEST performing universities as there is not enough to go around (in his words). So they are investing in those students and therefore typically more interested in their applications.