I like how they refer to "a reasonable excuse" for travelling, rather than a reason or justification or requirement or exemption. I assumed that was just the Mail, but no, the new government forms and guidance (for travel 29/3 and after) use the term:
You must complete this form to confirm that you have a reasonable excuse for leaving the UK to travel abroad...
Reasonable excuses for international travel include: •work•volunteering•education•medical, or compassionate grounds•funerals•leaving the UK if you do not live in the UK, and have only been here temporarily•allowing access to parents with children who do not live in the same country.
Find out more about the reasonable excuses for international travel...
You can fill in a freeform excuse if your excuse is not listed. 