All passengers leaving UK to travel abroad from Monday 8 March must download a form from the government website declaring why they are leaving UK and must provide evidence of this, such as a letter or official document.
Passengers without the form will be fined £200 and stopped from boarding. You must print a copy of the completed form or save it to a mobile phone or other device to show as proof.
Here is the link to the form:
https://www.gov.uk/guidance/coronavirus-covid-19-declaration-form-for-international-travel