Most universities require that you have completed a minimum of 2 years employment post-leaving school before undertaking an Access course.
In terms of what Access course you should do - find which universities you are interested in and which degree interests you the most and then look at their entry requirements - not all universities accept Access diplomas.
Once you find a university that does accept Access diplomas, find out what units they require you to study - then get in touch with the admissions department for that university and either ask which Access diplomas they accept, or tell them which Access course you were considering taking and they will say if they accept it or not. Generally, most universities will accept any Access course that includes the units (and the right amount of credits studied) they ask for, but always check.
Access courses usually take a year (full-time study) or two years (part-time).
IMO, you're only 17, you have PLENTY of time, if you can now afford it why not start your A-Levels somewhere else in September this year? That gives you far more flexibility in terms of uni choices as all universities accept A-Levels. I would definitely look into re-completing your A-Levels (from scratch if necessary).