In terms of whether you can sit exams externally i.e. sit exams without going to college. It really does depend on the college, most colleges will let you but often it is more expensive to do it this way than to just attend college. Because I don't know about other colleges but for me, being a full time student at my college was free and all you had to pay for was £45 enrollment fee each year for as many subjects as you wanted so just under £100 for the two years. However, at my college it costs (I've heard) £200 for each block of exams you sit, so you can sit all your exams in the first yer for a fee of £200 but to sit the second block of exams in the second year you would have to pay another £200, so £400 for the whole two years. But again this varies between colleges, so it may be worth checking on their policies, maybe go to an open day or email admissions into your college? They could give you answers.
AS Levels are the first year qualifications of A Levels. So basically, A Levels are done over two years, year 1 and year 2. The first year you sit AS Levels and get grades in those and the second year you sit A2 Levels and get grades for those, they then combine the grades you had at AS Levels and the grades you had at A2 Levels to give you your final grade for your A Levels. I hope that makes sense.
Personally, I would not recommend sitting A Levels externally because A Levels are very difficult and even more so when you sit the exams externally I can imagine. When you are in college you will get help and advise from tutors and lecturers and if there is anything you don't understand you can ask them to explain, whereas if you sit externally, if there is something you do not understand you wither ride it out, which could potentially affect your grade in the end or get a tutor which is adding to the cost. Additionally, when you do eventually apply to go to university you can get all the help and advice from tutors and its so much easier to complete when you have your lecturers providing your reference and predicted grades etc. At least I would have thought. I mean correct me if I am wrong because I have never sat an A Level externally but I think the best option is to go to college personally. What degree are you thinking of studying ?