London

Heathrow is far from central London, but the Heathrow Express closes most of the gap: trains every 15 minutes take 15 minutes to Paddington. From there the Tube or a taxi puts you in Westminster or along the South Bank, where Big Ben, the abbey and the river walk to the London Eye can fill a few hours. London is the one city here that rewards picking a single neighborhood over a checklist, because crossing town eats time. Check before you fly whether your passport needs a visa or an Electronic Travel Authorisation to pass the UK border. Heathrow's security lines and long walks to gates mean returning at least three hours before departure is sensible.