You can find qualified tour guides at
http://www.britainsbestguides.org/
But expect to pay more than £200 a day, and at least double that for a vehicle.

There's no reason you can't plan it yourself. Most museums offer guided tours, and there are plenty of walking tours available to get some more local information.

If the underground is a bit too hectic, you can get around easily enough in buses, taxis, boats, trains (at least outside rush hours).