I think AFPEX is actually very good but one needs to use it for what it is good at. The huge majority of flight plans filed by UK GA are
VFR ones - either going abroad (mandatory) or used in flight training or when flying in remote areas or just wanting to file one for fun. Previously, NATS had a large # of people whose time was spent mostly taking these by fax or phone or some other system, and entering them into the AFTN while adding addressing information out of some VFR addressing book (AIUI). Regardless of what one thinks on the subject of what services should be provided to VFR GA which already enjoys freedom from route charges, it was IMHO plainly only a matter of time before somebody decided this staffing is not an efficient way to spend money.
What I think NATS did wrong is that they delivered a very powerful tool which could be mis-used (not that I have tried but from what I see it certainly can be) and they then had to put the silly security around it.
If they had simply created a two-tier user base (one being airfields/ATC and the other being aircraft owners) and on the latter they limited the access only to specified aircraft tail numbers (which for most people would be their own plane, or a few others if it's a school) they would not have needed the security.
And for VFR, AFPEX works fine. You get no ACK message so that is no different to the old VFR flight plan filing systems where - despite having filed it via some apparently friendly human interface - the only way to check if it actually got anywhere was to telephone the departure or arrival aerodromes! That's just the way VFR flight plans work.
And IIRC the DOF/ parameter was never properly supported so if you tried to file a FP say 2 days in advance it would quite likely never get anywhere. Homebriefing incidentally deals with DOF/ correctly, by retaining the FP in its computer and injecting it into the AFTN only on the relevant day (
AFAIK).
It is for IFR, admittedly a much smaller pilot population, that AFPEX is potentially a slightly problematic tool, due to the lack of a slot reporting mechanism. It is however super in the way you get back the ACK or the 'fail' messages - in just seconds, allowing you to resubmit fast; a lot faster than with Homebriefing where the filing can take a few minutes. However, with good route generation tools this is not a problem.
If you file a VFR FPL via another method, whether homebriefing or your friendly local ATC, neither London Info nor Heathrow FBU would easily know who to send the DEP message to, would they? IFR is easier as it just goes to IFPS.
I am sure that this one has been solved, because Homebriefing has been around a few years now, and I know for sure a lot of users fly out of strips. Would the DEP message not go to the same addresses as the original FP? I reckon sending it to the destination only would actually do.
What exactly happens if you get airborne to say France without a DEP message? Does the FP get deleted after a certain time? And what if the DEP was sent to the destination only? If there is auto deletion, how is this managed - do the FPs get stored on some computer?