“From Monday 8 March, anyone who enters “a port of departure to travel internationally” without a completed form will be committing a criminal offence – even if they are legally entitled to travel.

Airlines, train operators and ferry companies will be required to check that passengers have a valid reason to travel abroad.

At present all holidays – at home or overseas – are illegal under the “stay-at-home” rule. The earliest possible date for going abroad from England for leisure is 17 May.

International travel is allowed only for a limited number of exemptions, including work, urgent family matters and medical treatment.
https://www.bbc.com/news/business-56295486
https://www.independent.co.uk/travel/news-and-advice/special-validation-to-travel-exit-permit-airport-b1812530.html
— LEAVING UK NOW REQUIRES A NORTH KOREAN-STYLE ‘SPECIAL VALIDATION TO TRAVEL’
https://www.independent.co.uk/travel/news-and-advice/declaration-travel-government-exit-permit-b1813070.html