I was in the same boat last year -- thought it'd be best to apply for illustration straight from yr 13.
And I tell you now, you'd be better off doing a foundation year as it will give you more options -- unis like you having a foundation year and they don't typically feel that A level is high enough standard to go straight to uni. + it get's you out the academic swing and into the creative one and it feels great.
So I applied to Bournemouth for illustration last year -- got an instant no. They want you to do a foundation year. (I know you can do a foundation there which pretty much gets you into their higher education courses)
Brighton, I got to interview but was the only one in the group interview not to have done a foundation so got a no.
Got a place at Lincoln and Falmouth for illustration and honestly, only having two places was disheartening and I felt like I had little choice in where I was going.
So I figured that the foundation wouldn't hurt and it's been the best decision ever -- compared to a levels, you do whatever you want with no restrictions to freely make art. My work has completely changed to how I was doing in 6th form and it's better; there's more of it because the lecturer encourages you to churn out work for interviews, preparing you for them as soon as you start the course. You're also doing art 100% of the time, not dividing your time between subjects; show's uni's you're dedicated. And with more time on you're hands, you get to do more personal work; which is also something unis want to see. And doing the foundation helps you because the lecturers know the good unis and courses and can help you choose what course is best (As in, I know of more uni's to apply too because 6th form didn't help since they were more focused on academic students, and I also know which ones I won't bother with) + you having a foundation that will put you up higher than those coming straight from a levels -- which is what happened to me and left me with barely any options -- and those people are the ones you're up against if you apply now.
However, if you do go through the whole process and then do a foundation, it's a good experience because you know what to expect at interviews and again it doesn't hurt to try first time round. Also, it gives you more time to talk to students and lecturers at interview (as well as other people applying to find out what they're doing) to find more and it gives you an extra year to decide where to go. You may also apply and have uni's offer you a place for deferred entry but ask you to do a foundation -- happened to a friend who wants to do fine art. You may even get a place without a foundation, who knows. But either way, it's all good experience whatever you do -- hope that helps!