Know of the best travel backpacks?

I'd like to buy a nice backpack for travelling around the UK - has anyone backpacked before and could recommend a good size, brand, etc?
I'll just need it for a week, by the way.

Hi Caroline! When I went to Europe last year, I did most of my shopping at www.travelsmith.com, but you can also get packs at www.ricksteves.com (I think this is the website...if not, google for rick steves). Good luck and have a blast in Europe!