I don't understand why the 2nd February is short notice? It happens on the same date every year. How long have you known the test is to be sat on that date?
Slough Mum's concern is that the outcome of her appeal may now only arrive a day or so before the test, whereas she might have had a little more notice if the appeal had gone ahead on Friday as planned.
SM, you don't have any official "rights" in this situation, but I think the Appeals team will be doing their best to rearrange the hearing for the earliest possible date. Clearly the current circumstances are beyond their control.
I suggest that you call them and ask what arrangements they can make for notifying you of the outcome of your appeal, because that is going to be crucial. Also, will they be liaising with Admissions to ensure that, if the Appeal is successful, you receive notification of the test venue and time as promptly as possible? Regular post is really not the answer to either of those problems, but phone and email could be. Both teams have tended to be reluctant to go down that route, presumably for reasons of confidentiality, but this may be an occasion when you need to ask for exceptions to be made.
I think it would be fair to expect that you and your DC should know the result of the appeal by Thursday at the latest, giving you both more than 24 hours to prepare for it mentally.