Anybody who wants to continue to file using NATS' current fax/phone system will not get a choice in the matter - its going to be AFPEx or nothing.
Are you sure about that? I tried to register for AFPEx and was rejected because I'm not UK resident, even though I have the need to lodge flight plans within the UK, for my international flights leaving the UK and returning back to Ireland, or onward to Europe.
The response I got back was that I could continue to use Heathrow and file by fax.
Are you sure Heathrow FBU is going to be closed? If it is, how are visiting and transit pilots supposed to file flight plans?