When I needed help with the flight plan etc, I used Lashenden (Headcorn). Helpful and experienced staff who would send the fax for you.
Now that you can do it all through Skydemon (apart from any prior notice required to Customs at the destination), anywhere with free wifi will do. I like Sywell, where you can get Mogas and a nice snack in the Pilots Mess (I have no connection).