It don't look like a problem as it is not paid work and they are not even paying the fuel etc.
However there may be a problem that they have actually bought a ticket?
I have often telephoned the CAA with strange queries and found them excellent. They will always help. Try your local office for a start. You will probably have to call Gatwick. Number is on CAA website.
You could check Lasors? Probably easier to call CAA
May caused an insurance prob!!