Fortunately there are foreign sites (Olivia, Netherlands AIS) that accept UK arrival or destination and allow you to file online without having to understand addressing or give a licence number.
It's a good question as to how much one should get hung up about this supposed limitation, but none of these sites offer (or guarantee) to accept flight plans wholly outside their own country.
I vaguely recall the Netherlands one actually says they don't. Olivia, I have never used.
It may well work in practice but I would not go off on a long trip abroad without being sure I can file a flight plan somehow, sitting in the hotel the night before.
Some pilots have more slack and they can mess around. I see them hanging around airports, getting weather, area notams maybe, filing flight plans by filling in the forms by hand.
I guess the online services (homebriefing.com being the only one currently running that I know of, but I believe there are others well higher up the price scale, Jepp included) are aimed at pilots who like to be all sorted by the time they leave for the airport and they just want to walk up to the plane and go. Historically this is what one paid a handling agent to do but nowadays it's possible to do it all oneself - in civilised parts of Europe anyway. And most GA airfields thankfully don't have handling agents (yet
)