If you are uncomfortable with the direct routing (say via Fenland to avoid the ranges in the Wash), you could route via Headcorn. They are experts at flight plans and the routing from Headcorn to Oostende is very simple. It also means that you only have to sit in your lifejacket / survival suit for less than an hour.
This site by Dave W might be of some help:
Going Foreign
Safe Flying,
Richard W.