No prob, Heidi... Actually, I'm going over to London for a few concerts pretty soon myself. I have a pretty extensive travelogue of a previous trip that might help you out with some of the questions here: Pissed in London: 2003
Heathrow's pretty out of the way, but the Heathrow Express can get you to Paddington Station in 15 minutes - 'course, it costs around £15. You can take the tube for a few quid and be in the center of town in around 45 minutes.
Hostels are pretty easy-going in London, so you generally won't have to worry about a curfew. I like Astor's Hyde Park Hostel for its location and social atmosphere, so it's the one I always wind up recommending (and staying at...) I know there are some bookstores in the area, but I don't recall any second-hand shops, but if you're in that neighborhood, you can check out the stores around Notting Hill. Didn't Hugh Grant have a book shop in that movie? Har!
You'll find a lot of bars w/ late hours around Oxford/Picadilly/Leicester Square, since it's a tourist/theatre hub, but there are plenty of other late bars around - just keep in mind that you'll often have to pay a cover after 11pm. Check out sites like Itchy London
for the good word on bars/nightlife.
The aforementioned Astor's (and many other hostels) are in the Bayswater area right next to Paddington and on the northern edge of Hyde Park, so they're the quickest in terms of getting in/out of Heathrow - one stop away by tube, hop on the Heathrow Express.. and there ya go.
have fun - sounds like a killer lineup. Say 'allo to Lemmy for me!