I self-taught Mathematics, Further Mathematics and Philosophy and came out with AAB (B in Philosophy). I'm now a Fresher at Royal Holloway studying Computer Science. It is definitely possible, but you must understand that it will be expensive - my exams cost close to £1k in total, and you will receive no guidance whatsoever - you must simply look at the syllabus, get some internet resources, join study groups on forums, and work very hard.