We'll be in England for about 10 days in July, and I am having a hard time with our itinerary. We will be renting a car the entire time (except for in London). So far we're planning 3 days in London. I'm also like to go to Bath (2 days?), the Cotswolds (2 days?), Oxford, Cambridge, Avebury, and the Jurassic Coast. Is this too much, too little? I'm also not sure if we should be moving around every few nights, or having a home base when outside of London (and if so, where?). Thanks for the help!
Except for Cambridge, the places in the country are in the same general area. I'd suggest doing London the first 3 days (get over jet lag), then renting the car for 7 days. You could take the train to Oxford (Avis is located near the station), see Oxford and then spend 2 nights in the Cotswolds, visit Avebury en route to Bath for 2 nights, then 2 nights on the Devon/Dorchester coast, and the last night near the airport. Save Cambridge for your next trip. Lots of other interesting places in the area like Stonehenge and Blenheim Palace.
I would highly recommend visiting Blenheim Palace which is on the edge of the Cotswolds right outside of Woodstock. You could spend 1/2 day there then 2 1/2 days visiting the small quaint villages of the Cotswolds, I believe 1 day in Bath is enough. If you really want to see Cambridge you could do it as a 1/2 day or a day trip from London, it's a 1 hr., train ride.
I have not been to Cambridge, but I would really discourage you from going there if you are going to Oxford. On such a short trip, it would be a mistake to go two places that are so similar. Oxford fits much better geographically. You would be far better spending an extra day in London (where there are a zillion things to do) or one of the other places you are visiting.
The Jurassic coast covers quite a large area. I'd suggest Lyme Regis as a center if it's the fossils you're interested in.
The county of Dorset is beautiful inland (I live there) and you could easily spend a few days in the area, driving around and seeing old villages, thatched cottages and lovely pubs!
The seaside resort Weymouth has a rather quaint, almost Victorian, feel to it.