No, you need to take the tube to Paddington, Victoria or other stations that serve that Bath area. You will need to take British rail from there.
Take the Metropolitan or Bakerloo underground railway line going North from London.
Yes. The Underground does cover much area, but you cannot take the Underground from London to Oxford.
There is no Underground station called "Charndlers Ford"
Take the tube (underground) to Tower Hill in order to get to the Tower of London.
As there are no direct trains from Heathrow to London Waterloo, it's quicker to travel by underground from Heathrow on the Piccadilly Line to Leicester Square Underground Station and then change onto the Northern Line to London Waterloo Railway Station.
Kings Cross
There is a tube (underground) station at the terminal which will take you into Central London.
Take a plane to London Heathrow Airport. Hire a car and drive the rest of the way (about 80 miles).
They are located on the south side of the river. Shouldn't be too difficult to get there from West London if you take the Underground.
There are several ways, train to Salisbury and local bus is one. If you just want to visit try one of the tour companies. They take you there and back from London
You can take the Eurostar from Paris to London St. Pancras.
Assuming you are already in London, you can walk, cycle, take a bus or cab, or travel by tube (underground).