All correct and well researched. However:
I have also signed up with NATS so I will be able to send a flight plan when returning back to the mainland and found these numbers to check whether the flight plan is accepted: 0845 6010483 or 01489 612792.
If you mean that you've signed up for the AFPEx online flightplanning service, be aware that the wait for an account is months. The numbers that you give are the (genuinely helpful) helpdesk for that service, and I think they'll only be able to help you if you filed through AFPEx. You can, however, without an account, submit a FPL by fax on 01489 612793, but you may need to address it, which you're probably not used to.
An easier way may be:
* to file all your plans via Belgocontrol (but I don't know if they'll allow you to file EGML to LFAT)
or
* to sign up for Olivia and use the French service to file that leg.
I feel like I need to apologise on behalf of the UK aviation community for the chaotic state of flightplanning in the UK.