Unless you've rented a car for your visit to London (not recommended). There are two options for most enthusiasts visiting England's capital: join a tour which includes transportation or get yourself to Watford Junction Station.
The station is connected to London's city centre at Euston and can be anywhere from about 15 to 25 minutes of travel time once you're on board.
From Watford Junction, the attraction runs its own shuttle every 20 minutes throughout the opening hours of the studio tour. The shuttle takes a further 15 minutes.
The shuttle requires you hold your tickets or confirmation for a timing and has a £3 fare per person, payable in cash or using a contactless card - that means no signing, swiping, or chip and PIN card payments.
There's a little more detail on Odyssean Travel's own England travel guide > LINK HERE
You can use an Oyster card to complete your London Euston > Watford Junction journey. Learn a bit more about the Oyster card here > LINK HERE