Stansted Airport is a popular destination just outside London. However, you can also rent a car or take buses. All of the major car rental companies have outlets at both Heathrow and Gatwick Airports. See the instructions below for how to get to UCL by car or public transportation. Finding parking in the area around UCL would be extremely difficult.
Prices are very reasonable.
UCL is between Gower and Gordon Streets just south of the station. They also don't necessarily get you to just where you want to go. The Airbus links Heathrow Terminals with many London hotels and some main line rail stations. The Airbus A2 line serves Russell Square via Euston rail station and also drops off at some West End hotels. There is also an all night bus service from Heathrow to central London.