It sounds to me like you should keep your job for now (even thought you dont like it, its money coming in) and look for a local part time course or a distance learning course - just do a websearch. Distance learning is a great way to learn if you are working, but you need to be disciplined.
If you have to leave work, you could apply for a student loan - try Barclays, Lloyds,or RBS.
Alternatively, why dont you get going by sending some illustrations to greeting card companies. I know that some of them actively encourage people to submit illustrations and they pay them royalties. This would help you get going and start your portfolio. Try for exampe, Phoenix Trading card company- they encourage submissions.