BA moved my flight under the medical policy before when I was told to isolate by NHS Track and Trace, so I’d imagine a positive test they will be helpful to move to get you home.

This is under the medical policy- and they can put the flight on hold and rebook once you are fit to fly- so no need to worry about the ‘can only change once advice’, that’s only relevant to changes due to schedule changes etc.

I hope in any case, such circumstances aren’t required and you have a good Christmas.