To be honest, you've more graphic work to show than I had when I got into the Cambridge School of Art (Anglia Ruskin University) in 2005. I was coming from an A-level art background with no graphic design experience whatsoever, and I still got in. Having said that I suppose there's more competition for University places now, and it's always best to be fully prepared.
I agree with Sunburn, that it would be a wise move to bolster your portfolio with some responses to theoretical briefs. A good book for this is The Graphic Design Exercise Book by Carolyn Knight and Jessica Glaser. It contains briefs of varying levels (beginner, intermediate etc) for logo, packaging, layout, and web design, amongst others.
Can't hurt to give yourself a rudimentary understanding of design theory - maybe do a couple of experimental pieces exploring balance, colour etc. But really, that's the kind of thing you'll be learning at university I'd have thought. They won't expect you to be fully versed in graphic design theory.
It's also worth having the names of a few well-known graphic designers to cite as sources of inspiration. When I had my interview I'd recently been to see an Avant Garde graphic design exhibition so mentioned names like Jan Tschichold and El Lissitsky. Seemed to go down well.
One thing they won't teach you at uni is how to use Photoshop (beyond the bare basics) so you've got a head-start there. And you've tried your hand at logo, web and layout design, so they can see you're enthusiastic and keen to explore multiple design disciplines.
Good luck.