How did you manage to leave at 12 ?? Unless you are 50+ years old it has been a legal requirement for either school or home education until the age of 16 for many years, and if educated at home the LEA would have sent inspectors to check you were being taught the curriculum.
Have you spoken to your local college ?? there are various funds available to help pay for courses and any material needed, the part time Degree I am taking has access to two different funding streams that will pay all but £100 of my course fees and give £250 for books etc, the college and university also have hardship funds which can be given to students who need extra help.
If there is a course you would like to take, but do not have the required GCSE's etc, look to see if there are other ways of qualifying as a mature student; technically I do not have the qualifications needed for the course I am taking, but after reviewing my work history and previous studies the tutors said I was more than capable of taking it.
Finally, the GCSE is so focused on the exam in most subjects that buying or borrowing the revision book will teach you enough to pass!!
The final option when applying for a job is to lie !!! I worked for Great Universal Stores for two years and in that time they never checked any of my references; HELL !! they never asked anyone employed to drive their lorries to produce a valid HGV or Drivers License !!!