HSBC do a professional development loan of up to £25k, but whether they consider a pilot a professional or not (and thus pilot training to be eligable for said loan) depends largely on the branch you go in to - I know the Watford one does and (one of) the Oxford ones doesn't. If you're looking for supportive banks, try the ones around the major training centres, as they're more likely to have some sort of pilot-related scheme. Also, a lot of the big trainers (Cabair, etc) will have their own deal with a specific branch which they make available to their own students - try contacting the schools.
There is also a (gubbermint supported) career development loan scheme (up to £8k, if I remember correctly), which several banks participate in (RSB and Halifax maybe?) - for more info on this, resort to google.
Another option in the loans department is a personal loan, which most banks do up to £25k - obviously, the amount they'll give you depends heavily on your credit rating and (I assume) income.
If you're under 25, there are a few bursaries around, but they're generally a few £k tops, and I imagine competition is fierce. I know the BWPA do one and I think there are a few others - again, google is your friend. Don't bother contacting the BHAB for this, btw, as they're not very helpful (in this specific regard).
Obviously, before you go into oodles of debt, make sure you have a very definate and well thought out way of paying it back! An underrated and oft-ignored alternative to bank whoring is to actually do some saving first...