Try Haggis.  They’re great!

There are tons of them all over England and London in particular. Once there, just head off to either Victoria or Waterloo train stations and see all the package tours and companies. Take all the information down and they have many different kinds of them. Most of them are over the Internet.
 
If I remember the ones I took I will send you the names.
 
One of them I took was called trafalgar tours. However this was a long time ago. I am sure there must be plenty now.
 
 

Sammohanty has good advice.  If you’re sure you want to take a tour (UK is easy to explore on your own) I’d suggest a 7 day, or shorter, tour and some time on your own.

Trafalger is a big tour operator out of London — kind of interesting tours since they attract English speakers from all over the world.  We had Australians, Malaysians, ethnically Chinese Malaysians, Israelis, South Africans, British ex-pat’s from Yemen and Americans on the one we took.  We liked just about everybody in the group, however, it was pretty much retirees, a couple of families and us (age 28 at the time).  The crowd wasn’t much fun for 20 somethings and a 23 year old solo female would have been “mothered” and “grandmothered” to death. 

I think Trafalger has tours that are targetted for the younger set so look for those.

I would encourage you to see England on your own.  Transportation is excellent even to small towns, and you speak the language.

Initially stay at a hostel in London. This would help you take daily tours to London city, Windsor, Oxford, Bath, stonehenge, york and stratford-upon-Avon, the birthplace of Shakespeare.
Then visit the countryside for overnight stays in Cotswalds, then move to Scotland and a few other towns.
 
I should be back in London shortly and pick up some brochures from the station and provide you the phone numbers.

I really like the company London Walks – http://www.walks.com/.  6 pounds now for their walks in London but they also offer excursions to the Cotsolds, Oxford, etc for a very reasonable price (at least it was a good deal with I was there). I believe you arrange your own transportation (so you can stay in the place longer if you’d like) but they do meet at the train station, so you can go with the group if you’d like. 

Keleti, How was Cotswalds?

I didn’t go on the Cotswolds tour with London Walks but I have been there. I went with my dad and we did a few walks in the area, which were really nice. You can walk through farmers fields, etc. I like the cream teas you can have in most places there!

Thanks everyone! I’ll look into the suggestions.