Hi,
If you want to resit your A levels, you have some options.
a) go back to college full time. ANY college would take you with a degree, though they might question why you want it. Expect to pay around £100 per yr per AS subject for tuition. You would have to put up with 16-18 yr olds though, so im guessing you dont want to do this. Some colleges might offer you intensive 1 yr retake programmes though.
b) go back to college as an evening attendee. Good news is that there wont be any annoying 16 yr olds, bad news is that it will cost slightly more (~£120-150) and your severly restricted in subject choice. Most colleges only offer things like English, Maths and a couple of other humanities like History.
c) Sit them as a private candidate. You do the work yourself via self study. For this you can use things like ICS etc though I would seriously discourage you from using these or just get the exam specification and teach yourself. ICS and other such companies are grossly overpriced (around £350 for 1 A level and that doesnt include exam fees) and the tuition they give you probably isnt all its cracked up to be (they send you a booklet of 300 pages or so for your £350). I am speaking from impression rather than experience about them so take this with a pinch of salt.
Anyway, you then have to phone up a college and get them to enter you as a private candidate which basically means you get to take your exams with their own students. Expect to pay a fee of about £10-20 per module, so that would be ~£80 for a full A level. Officially the exam board fee is £10 and the college will charge you an admin fee of whatever they like, tho it is usally low. Downsides of this are: Colleges wont enter you for exams that their own students arent sitting, so if you want to do edexcel history then you have to find a college that does that spec. A college doing AQA wont put you for edexcel. Also, coursework and practical components are nigh on impossible to arrange since it is a lot of extra work for the college. This means you will be restricted to doing just written exams at most centres, excluding subjects like physics which have practicals and languages which have orals. You can find colleges that do these if your desperate, but they may charge you extra and you have been warned how hard it is to find them.
Im in a similar position and I would reccomend option C, its the cheapest and since you have a degree you should be able to teach yourself fairly easily. If you email AQA or Edexcel for a private candidates information book they should send you a pack of all colleges that accepted private candidates last academic year. This isnt vital, since you could just randomly phone colleges up but it might direct you a little better.
Im sorry if my grammar waned a little bit or I rambled but Iv done so much research on this I got bored by the end of typing it! Any other questions PM me and ill try to help out further.