As someone said, keep it brief and polite. No need to go on about your credentials as a 2000AD fan - the sample artwork you send should do that for you, and include some recognisable 2000AD characters. Dredd is usually the favourite, but some others - Rogue, Sin-Dex, Lobster Random, Dante, whatever - might catch their attention and show that you actually do read the comic.
Send them a page or two of sample strip work, not just pretty illutration pieces from your portfolio. They want to see that you can tell a story.
If you've got any previously published stuff, be sure to mention that in the covering letter and, if possible, send them a copy. As well as good artists, they've looking for people with a proven track record in hitting deadlines and working to an editorial brief.
Good luck with your submission.