TAP Portugal, the Portuguese flag carrier, offers regular, direct flights from London-Gatwick to Lisbon's Portela Airport. The duration of this London to Lisbon flight is two hours and 40 minutes. British Airways also offers direct flights from London-Heathrow. But if time is a major factor in your travel plans, then you are better off with the TAP flight from Gatwick, as delays are much more frequent at Britain's largest airport.
