OK, you can get most places IN London by subway "Mind the Gap".
Combine trips
When you go to Stonehenge, take the train from there to Bath also. It makes a good full day.
Windsor Castle is a must
The British Art museum off Trafalgar Square is a "gotta see" and the Museum of History (rosetta stone, greek ruins, etc..) is walking distance from the Tottenham Sub station, behind the downtown/uptown YMCA. These two can easily fill a day.
When I was there, as I understand it still is today, everything was the equivalent price in lbs as it would be here in dollars. So a $10 meal here was £10 there or about $15-$16 (2002), so look at the exchange rate and plan accordingly. London has every type of food imagineable, right off the main streets.
Enjoy yourself, travel AS A GROUP, do not walk alone!!!!
You'll have a blast. Remember, the British ruled the world for a while and stole most of the really cool stuf for their museums.
The art in the Trafalgar Square museum has such depth and color, it's almost impossible to believe some of it is 400-500 years old.
If you have a digital camera, make it a small one (pocket or purse size) that uses AA batteries. (you will be less willing to haul a large SLR type camera around). You should have at least 512mb - a gig worth of cards and at 3MP size(perfect for almost all shots), you can take about 500-1000 shots with that.
Have a blast....