Have you spoken to anyone from your potential sixth form/college where you plan on studying your a levels? I would think different places may have different admissions rules and routes, so it's probably best asking their a level team to find out the easiest possible way to get you enrolled.
At my local college, for example, you can simply submit an application online and can apply at age 18, and you go through the usual application process. But I think it's best if you get some formal information too.