How can i travel to london fron USA? I am a have a non-immigrant visa from USA?

You can fly, take a ship, or swim.

You can travel to USA as a tourist, but you can get a job. With the visa A you can visit the country.
Book a airplane ticket and hotel where you want to stay , then give the custom registry at their border/airport and don't do any suspicious.

Depending on what country you are from (obviously not the USA), you may need a visitor permit/visa to go to London. You would apply to the UK for a visitor permit.

Easy, go down the airport and tell them you want to fly 'standby' on the next available flight. Someone won't show up and you can nab their seat for cash like $100. You don't even need a visa for the UK just blag your way in and join the party. We are much cooler than the US.