@WearyAuldWumman this isn't the same situation.
The OP has left the school's employment and has taken sick leave to bridge the gap between the end of term and her leaving date.
In this instance, the school is entitled to get information, resources etc from the OP that she possesses in the form of a handover. How that is done is negotiable, however. It sounds like all that needs to be completed is a simple email with the outstanding information from the OP.
The fact that she is refusing to do this and refusing to let the school know when she might be able to give this information, is leading to unnecessary stress on both sides.
All the OP needs to do is either:
a) just sit down for an hour and write and send the handover email
b) send an email to the school to let them know she is too unwell to complete the handover at present but will send it in time for the first day back in January.
I don't think receiving an email from the school asking when she will be able to complete her handover and a couple of automated texts about a Christmas lunch is union-worthy, frankly.
I'm all for pushing back against unreasonable demands, but I don't think the school are being unreasonable here in requesting a handover from an employee who is leaving mid-year and who has information staff taking over genuinely need in order to effectively do their jobs.