One problem is that the over the top security prevents you accessing Afpex from an internet cafe computer.
This is OK for me - I have been carrying a small laptop, with GPRS/3G, for years - but not many pilots do that. Unfortunately far from all internet cafes offer wifi or a cable connection, to enable the laptop to be used.
Homebriefing has a simple login/password access. You can still use HB to file a flight plan for anybody else's plane but you cannot query the flight plan database like you can with Afpex, but why would any pilot want to?
If somebody ex-Heathrow is looking to set up a HB-like service, then I suggest HB is the starting point to look at. Allegedly, their software is an off the shelf product from the same company as the old ais.org.uk Notam site (Thales??). I never had a usability problem with HB.
The general thing to get right on any website is to minimise the amount of data transferred. This is vital for users on GPRS/3G especially if on PAYG. But if you pay some whizz kid to design a website, you get a megabyte here, a megabyte there, and everybody thinks they got value for money. The end result is crap, and usable only over ADSL, or over 3G if you are on an inclusive roaming tarriff (not many of those under about £40/month) or somebody else is paying. HB have a reasonably simple site which doesn't transfer much data - I guess a few hundred kbytes to get a job done; I don't use it for notams, for which I use the NATS site. But even a few hundred k is way too much; the entire user interface of HB can be done with a few k. Basically all you need is a very simple form like the Eurocontrol flight plan validation site.
As regards addressing, I think some people make this appear harder than it is. For IFR the FP just goes to the two IFPS addresses. For VFR, it goes to dep, dest, (a nearby airport if either of these is non towered) and the area FIS addresses. But if you are VFR, few people along the route care much about you anyway.