If you're planning to spend time in London on a budget, be sure to look at these 47 cheap and free things to do in London. Also great for families with kids Last minute summer holidays www.hkoffers.com
Starting place for the Harry Potter tour! The tour takes 1-2 days to complete, but it might be good to stretch it to three to actually do some of the attractions that you pass on the way (like the Tower of London or the Globe). Best part, it's free!