I've got a five-hour layover in London Heathrow airport coming up as part of a long-haul transcontinental trip... is that enough time to leave the airport? I don't want to chance missing my connection, but I'm also not very excited to sit around by myself in Heathrow for five hours. How long a layover do you realistically need go into London? I'm on an American passport so I don't need a visa. Also, if it is enough time, what's the most convenient way of getting into the London?