By way of update, as it may be of assistance to others, after failing to get the number ported with A&A, I signed up to Voipfone and asked them to port my number (on 12 July 2023), within an hour of sign up with my request to port the number (had to complete a few forms) I received an email back from them confirming that my request to port the number was successful and the port would happen on 18 July 2023. Today I received a confirmation that my number had been ported and it now works.
I am not sure who is at fault here and why A&A's initial request to port the number was rejected (I don't think they subsequently attempted a port after the first failure as they stated they are charged £15 for each request and simply continued to state that they can't port an inactive number).
At one point when speaking to Plusnet on the phone, I was informed that the number goes into quarantine for 10 days after the switch to FF and I should attempt to port again after this period (which is when Voipfone did). However I am not sure how true this is, what it technically means or whether this could have been the cause here.
In any event, the above shows a port can be done and hopefully whatever the issue was it can be ironed out in future for others.
