Just to be awkward and to quote from the rules I suspect their trip is allowed it's allowed under the following, but they might need a letter from Mum or Dad

Work travel allowed
"Essential travel for business or official work purposes where it is not reasonably possible to complete that work from home.
This includes but is not limited to essential work or returning overseas having completed essential work, in relation to critical national infrastructure including the national rail network, national security or diplomatic purposes, and elite sports competitions.
Recommended evidence: employer’s letter, professional ID card, confirmation from sports body or evidence of participation, diplomatic mission letter, etc."
www.gov.uk/guidance/coronavirus-covid-19-declaration-form-for-international-travel
As has been pointed out by pp Greece isn't on the red list so managed (hotel) quarantine on return to the UK will not be required, self isolation/test and release etc..
..but yes, all that said, the "optics" aren't great...