The Consumer Credit Counselling Service is a registered charity so, unlike the many 'debt advice' services offered by commercial companies, its advice is impartial.
They've got an online system (where you simply answer a questionnaire) which recommends the best course of action for individual circumstances:
http://www.cccs.co.uk/
National Debtline is part of the Money Advice Trust, which is also a registered charity. They've got loads of information and advice on their website. They also operate a confidential telephone helpline:
http://www.nationaldebtline.co.uk/
Chris