isn't it a bit overkill to have a discrete AFTN address for each user?
Sure is, but that's the deal and I don't think it will bother anybody.
Olivia is OK but is not supposed to work for flights wholly outside France. The NATS site should work for anywhere, so you can be in Bongo Bongo Land, in an internet cafe, and you can file a flight plan to the Peoples' Democratic Republic next door
Only the one website to remember, only one login, only one interface.
Pilots have recently been using Homebriefing.com (based in Vienna) for this purpose, but they have just restricted the max # of flight plans wholly outside Austria/Switzerland to 10 per year. I have just renewed my annual HB sub and curiously they don't appear to say how they will charge once you go over the 10. Maybe they just reserve the right to charge for them, in case of people filing loads of flight plans (which I am sure some users are doing; the 37 Euros is a great deal for unlimited flight plans).
What I do wonder about is what will happen with the NATS site when some UK pilot enters a really funny route, with the names of various villages. Some people do actually do that. I don't know how Heathrow used to handle that. Maybe they just typed in whatever the pilot wrote - what else could they do? I have never tried that with Homebriefing and I am not about to - I always navigate fully IFR even if flying VFR. But this could cause a problem if the NATS site does any waypoint validation. OTOH pilots are supposed to get notams and the ais.org.uk Narrow Route Briefing never allowed weird route descriptions anyway; you have to stick to the ICAO route description.