Day trip from London (small group tour operator) - need recommendation

I've done 3 bus daytrips with Evan Evans (plus their day trip to Paris) and have nothing but the best to say about the operation. My favorite was Windsor Castle/Bath/Stonehenge.

Look at London walks website. They do day trips, using the train - depending on which trip it is there may be a bus waiting at the destination station for onward travel. www.walks.com.

I've done several of these days trips, groups have been from 8 people to about 30 - you don't have to book.

I second London Walks. Definitely look into their offered trips.

We just did a day tour of Oxford and the Cotswolds with http://kookycotswoldtours.com/

Not the cheapest but it was a great day for my family. They know all the history, best places and shortcuts to make it a well-packed experience.