900 pilots employed by Virgin Atlantic and who are members of the Professional Pilots’ Union are set to work to rule from later in the week in a dispute about recognition for the Union. I understand that this is the third year of negotiations which have not come to anything to date.
It is likely that some flights may be cancelled at short notice, for example of pilots would breach a term of their contract by operating the flight. I suspect they will decline to work non-contractual overtime too.
If you are planning to travel over the holidays and have a reservation on Virgin I would keep an eye on the flights status.