You can join an ancountancy firm after your A levels and study fot the AAT and then onto ACA etc. AAT is basically a qualification that school leavers have to do before they can get the ACA and become a fully qualified accountant. Graduates don't do the AAT, they just do the ACA. Have a look on PriceWaterHouseCoopers or KPMG website and search for school leaver opportunities, there will be a lot of information about it there.
Hope this helps,