Well I have lived in the UK for a few years now, and I'd like to say: don't make the mistake of spending all of your time in London! It's a great place but there is so much to see here beyond London....
for budget flights from London to the rest of Europe, I would recommend the following websites:
www.ryanair.com, www.easyjet.com, www.go-fly.com
Good luck and ENJOY!