Access courses are designed for people who have been out of study for a few years, so I agree at 17/18 you would be better off looking at AL’s, either as a private candidate or at a sixth form that will take you.
Whilst uni’s may state that they accept access courses, the reality is that they may not accept the specific one that you are looking at for ACCYFIN or whatever similar course you want elsewhere. The only way to be sure is to contact her Uni in question with specific details. Otherwise their websites tend to keep things vague. AL’s may keep more doors open.