What an amazing tour. If you are visiting London, this trip is a must because you get a sense of the "real" rural, historical England.
The guide was knowledgeable and amusing and it gave context to everything we saw. Warwick Castle was particularly interesting, with waxworks and live actors combining to provide "an experience" rather than just another castle. If you know and like British history, I really recommend this tour.
I probably could have spend half a day at each place, but I'm sure it is the same with any tour. The price covered entry into everything, and each is quite expensive so it's excellent value for money too.
There is quite a bit of walking especially in Oxford and to enjoy all three places you really should walk around - there were complaints from a few of the chubby Americans. - With so much to see, there is very little time for shopping or souvenir hunting if that matters to you.
I liked the fact that this tour attracted adults and not kids.