Kids take GCSE exams in a number of subjects just before they leave school at 16+. The courses are usually two years, but you can take one-year, part-time GCSE courses at community colleges and further education courses, either daytime or evening.
A levels are taken in two parts, AS and A2. Each part is a one year course, and an AS is worth half an A level. Kids tend to do them if they stay on at school after 16. Again, as an adult you can take them as part-time courses.
A route you might want to consider is that of an Access course. These are usually run by colleges in collaboration with universities and designed to prepare mature students (which is what you would be) for university entry. They can either be part-time or full time, and usually 12 to 18 months in length. Success rates are quite high. You'd need to contact the individual universities about them or, as above, ask at the local FE colleges what's available.
Another idea, longer but more practical if you need to earn, is the Open University. These are modular, distance learning courses, but you get plenty of support, either online, by telephone or in regional tutorial groups. You can start at a very basic level and still earn points towards your degree:
http://www.open.ac.uk/