Homebriefing is a very good flight plan filing service, which you can sign up to immediately. They charge about 40 euros per year and that gives you 10 flight plans, except flights which start or end in Austria or Switzerland in which case you can file as many of those as you like.
Many European pilots have been using HB for years.
For IFR flights there is
EuroFPL which is currently free.
I believe that AFPEx is available only to pilots who can provide a UK address.