Take hand sanitizer, ziploc bags, oatmeal and ramen noodles. 

Make copies of all your credit cards and IDs front and back, passport included, leave copies at home w/ parents and take copies in your suitcase. 

Learn the bus system, it's significantly cheaper than taking the tube everywhere.  

Eat GREAT, CHEAP sandwiches and wraps at Pret a Manger. 

Don't waste a minute, wake up early, and see as much as you can!