European Tourism Association - a membership organization, primarily of tour operators - CEO Tom Jenkins said in an interview last week that in the event of a no-deal Brexit, if EU countries choose to treat UK citizens as non-EU-citizens - not enjoying full freedom of movement between EU countries - that could add 90 seconds to every airport border check of every UK citizen.
The UK government is encouraging UK citizens to go ahead with booking flights after the 29 March official Brexit date. But unfortunately that may simply reflect wishful thinking.
Stay tuned.