Computing is indeed one of the rarer A Levels. Only 5,610 candidates took the computing A Level exams in 2007 which was fewer than the number that took Spanish. Last year Neil McBride, a principal lecturer in computing, from De Montfort University criticised the computing A Level as being tediously boring with a curriculum that was tired and dated in an article published on the
British Computer Society website. I haven't really investigated the computing A Level curriculum recently but if there is much truth to what Neil McBride says then it could partially explain why very few colleges still offer the computing A Level course, and instead offer the ICT A Level or vocational courses. I have a suspicion that independent schools with 6th forms are more likely to offer a computing A Level than state run further education colleges because the computing A Level is perceived as being of higher academic calibre than the ICT A Level or vocational courses. This could work to your advantage if you want to sit the exam because independent schools tend to be more willing to let private candidates into their exam halls than state schools and colleges are.
If you have no success in finding an exam centre that offers a computing A Level then there are a number of alternative options.
1. Check out the ICT A Level. You might find it more interesting or relevant to the real world than the computing A Level. According to Neil McBride the ICT A Level curriculum contains programming although I haven't compared the quantity and depth of programming between the computing and ICT curricula.
2. Enquire at a further education college. Some colleges are willing to allow under 16 year olds to study A Level courses. They usually study in evening classes alongside the mature students rather than in the day classes for the 16 to 19 year olds for some reason only known to the college. Most colleges now offer services to 14 to 16 year olds but they are generally vocational rather than A Level courses. Free college education is officially only available to 16 to 19 year olds, so if a college allows someone under 16 to study an A Level course then they will probably charge the same fees that adults have to pay.
3. Change to a vocational course. Some vocational courses such as the ECDL or basic website design are probably beneath your ability but a number of advanced vocational courses exist such as Microsoft, Linux, and networking certifications. They are offered by specialist course providers rather than colleges so fees are charged. However, anybody can take them without any previous qualifications such as GCSEs and I believe there is no minimum age limit. These advanced vocational courses are recognised by industry and employers who sometimes value them more than academic qualifications. You might find them more interesting and relevant to the real world than A Levels so they are definitely worth investigating.