Yes. The Underground does cover much area, but you cannot take the Underground from London to Oxford.
There are 13 underground lines, 470 stations, and, at peak hours, 525/527 trains running at the same time.
The Tube
Congestion throughout greater London and the City of London. 6 separate railroads came into Greater London but only one led directly to the City of London. This meant the trips had to be continued further into the center by road so they proposed an underground railway to relieve congestion. cheers, Skunkfarmer... New Hampshire USA
The only railway under the river Thames is parts of the London Underground (tube).
Heathrow
You can't. The London Underground and London Buses only go as far as the suburbs.
One only and that is called london City
45% of the overall network is underground. Nearly all of the lines run underground through the city centre and overground in the suburbs. The busiest stations tend to the be the ones which are completely underground. The Victoria Line is the only line which is completely underground (with the exception of the train depot at Northumberland Park). All the others run partly underground.
The Circle Line Unfortunately this answer is wrong! According to Transport for London the only tube line to connect all London Underground lines is the Jubilee Line.
The Central line only.
Walk. It's only about 200 yards
London is the capital city of England and the United Kingdom. The only City State that I know of is the Vatican City.
No that's only in Indiana Jones
London, but only by 400 miles.