The 550D is a new model and has good reviews so far. It has more MP than the 450D, but that's not necessarily a good thing when it comes to noise at high ISO settings.
I'd have a look at the 50D, I think you'll find it's a much more robust camera than the 550D, but consequently heavier and bulkier.
If you decided to go for either of these cameras you would have change left out of £1000 to put towards a good lens. As far as lenses go all the Canon lenses will fit on the crop sensor cameras (450, 550, 50D etc) but don't try and use an EF-S lens, dedicated for the crop sensor cameras on