Stonehenge is worth a visit, we camped near there the summer before last for a weekend and went for a trek round the stones. Unfortunately the conservation nazis won't let people get up close and personal with the stones unless you go on the solstice.
You do realise that Stonehenge is not in or near London, don't you?
Edited to add: If you are travelling that way, Avebury is worth a look, it's a site with a full circle of standing stones that you can touch and has a decent pub.