I'm studying 3D at Bournemouth University, they have a neighboring school called Art University College of Bournemouth. Some of my classmates are from there, and they have really impressive illustrations. Check that school out.
Some of the parts on my course do cover digital illustration, however I am taking Games Art and Design at Norwich University of the Arts, so there is large degree of 3D involved. However in the second year your given a large amount of freedom to pursue your own tastes in media, the first year is there more or less to let you work out what works for you.
Don't worry about not being good enough to get in. Seriously! Most people will be coming from A-level and won't have as much experience as you! So, mister, that should be the least of your worries! You'll probably be better than everyone else and they'll be so jealous of you and in awe! (I know I am anyway)
Anyway, I think Digital Art courses specifically might be hard to find but I agree with Avenvia in that you should look through UCAS. Or you could try here - [link]
And your portfolio will be amazing. They'll want you on their course, they'll probably be bribing you to come! HAhahahah. I don't know. But listen! I know it's hard, but please don't worry too much about the quality of your art. People graduate in the third year drawing the sort of stuff you do
First, check UCAS: [link] is a search for "digital art" as an example. I would recommend trying variations of that to make sure you explore all options.
Second, check the league tables: [link] is one example for art and design.
From there, get a list of universities that do a course you're interested in and that are ranked well on the table (though the table should be treated as a rough guide anyway, not a definitive list of the best to worst). Look at their websites and check their entry grades, modules and the overall specifics of the university.
Ask your teachers if you have doubts about the quality of your art. Most universities will also ask art students to submit a portfolio of work - it's never to early to start building one up. Make sure it focuses on what art you want to create, but also shows a range of skills and techniques.
Also, and this is pretty important, some art courses (photography and fine art are the only ones my friends applied do, and most of the top universities for these followed this pattern) will want you to do an art foundation year in college after your A levels before you apply. Look for this on their websites, because a friend of mine decided he was so wonderful they'd let him in without one and got rejected for it.