London is one of the busiest transport hubs in the world, meaning there are plenty of opportunities for cheap flight bargains. Heathrow is a monster airport which in true London style has grown organically and now has four terminals (a fifth is being planned). It has two tube stations. Gatwick is also a large airport, but much smaller than Heathrow, followed by Stansted and Luton. All domestic flights and those to EU countries from London attract a £10 departure tax. For flights to other international destinations you pay £20.
For the first time since the ice ages, Britain has a land link (albeit a tunnel) with mainland Europe. Two services operate through the Tunnel: Eurotunnel operates a rail shuttle service (Le Shuttle) for motorbikes, cars, buses and freight vehicles between terminals at Folkestone in the UK and Calais in France; and the railway companies of Britain, France and Belgium operate a high-speed passenger service, known as Eurostar, between London, Paris, Lille and Brussels. London's main rail terminus for Europe is Waterloo International.
Even without using the Channel Tunnel, you can still get to Europe by bus - there's just a short ferry/hovercraft ride thrown in as part of the deal. Bus travellers arrive and depart from Victoria Coach Station, about 10 minutes walk south of the Victoria railway and tube station.