With reference to AFPEX, as previously mentioned you need a UK address but also require a UK CAA license number to verify the details for setting up the account.
Not quite. I have checked that a UK resident holder of a FAA CPL/IR can be signed up. I guess the verification might take longer... mine took about 2 months I think. I do actually have a CAA PPL/N/IMCR but I didn't supply those details.
Olivia works differently, I believe, to AFPEx. The former has some kind of human element, though perhaps less than most people think. The latter is a straight AFTN terminal; you can send free text AFTN messages to your mate working at some airport in Mongolia
With Olivia, they can do "stuff" to drop flight plans they don't like. AFPEx has no obvious means of doing that; the FP you transmit is going to go where it is addressed...
I really like AFPEx. It is 99% very good. The other 1% is stuff like copying one's inbox to email, which would be brill for slot notifications etc etc and would cost NATS nothing to operate .... once they paid the vendor £1,000,000 to implement the extra software
I am not aware of any "GA" flight plan filing facility which is officially not geographically limited, other than AFPEx, EuroFPL, and Homebriefing.
Of course there are other, upmarket, services which do various level of preflight stuff. One German firm starts at 35 euros per flight leg, for which you get the Eurocontrol route developed and I think some kind of weather pack. The you get Jeppesen et al who tend to work around the 3-figure / month area.