Probably depends on how much you like your current employers. I did a similar thing 12 years ago and decided to give three months notice instead of one month - but I'd known I was accepted on to my chosen course a year before it started.
If you decide to start your career change sooner it's worth looking at Career Development Loans -
http://www.direct.gov.uk/en/EducationAndLearni ng/AdultLearning/CareerDevelopmentLoans/index. htm
It's obviously better if you can save in advance but if you just can't wait to change career, they are an option that is cheaper than a traditional loan, which is what I did - although I then had repayments for five years, but it was worth it, so much so that I'm thinking of changing careers again and getting another loan to do it.