Why are you going through a tour company? When I taught at an American school in Saudi Arabia, our teachers planned these outings through travel agents. Our secondary students went on extended field trips to India, France, the UK, Japan etc.
The use of a travel agent allowed our school to tailor trips to each specific group. The more people who went, the more free hotel rooms and plane tickets they got for accompanying teachers. (Parent volunteers also went - but they paid their own way.)
The travel agents booked charter buses and hired private guides for touring. They also booked hotels, shuttle service to and from the airport, and plane tickets.
One possible source of help for identifying places to visit would be the American School of London. Their website may be found at:
In general, there are American schools in most countries throughout the world. I myself taught in Saudi Arabia and Lebanon. As a student, I attended American schools in Ghana, Thailand, and EL Salvador.
The local school could prove to be a useful resource if you can find someone who'd be willing to correspond with you.
Another resource would be the London Tourism Guide book. This used to be a free publication complete with an A-Z map guide but unfortunately it's now out of print. The full text may be found on-line at: [url]http://www.londontourist.org/[/url]