I don’t know about brands and styles, but I suggest you go to your nearest outdoor camping store and have a look at what’s available. Most large backpacks come with an attached daypack that you can zip off and use just for light travel. There’s myriads of regular backpacks to choose from these days and it’s just a matter of how much you’re willing to spend on them. If it’s just for london, you won’t need a full on backpack with attachments for camping gear. Just get a standard backpack that won’t be too bulky and a good idea, is getting a backpack that has zips you can push together and put a small lock through for extra security. The secret is, think about what you’d need to put in your daily backpack, and make sure the backpacks you look at can accommodate all your needs. I wouldn’t pay too much attention to brands if I were you. Especially since you’re not going to be hiking. Just make sure it’s sturdy and doesn’t disintigrate upon touch and you should be fine! However, if you’re planning on doing more extensive travel in the future, it might be worth investing in something more substantial so you don’t have to buy a new one in the future.

I wouldn’t even worry about getting a "Special" backpack for only 2-3 days. I’ve got just a regular Jansport schoolbag that does just fine. Just get something with padded straps and back and a heavy, durable zipper that will last.

Suzanne is saying, "do it the same way you would at home." This makes eminently good sense on most topics here: Wear, pack, use the same thing you would take in the US on your vacation if you went backpacking (or in most cases, even if you were just satying in a hotel). Europe doesn’t require special shoes, shirts, pants, dresses, toothpaste, deoderant or anything else, and if you leave something behind you can find exactly the same product in most places in the EU with the exception of Denmark, where the natives try extra hard to be different.