Frightlevel
I think you mean your weight is 1999KG?
Yes prepared to be dumped out of airways as you put it but plan to control the descent so that you stay in CAS as long as possible and pref until you can get a RAS or two way with your destination field.
Never had a problem joining CAS at some point ahead and being turned away because there was no plan accepted and in the system.
I usually just file it IFR DCT XYZ with an estimated time for XYZ then pick up the clearance through a military controller or failing that with London info Exit point is usually DCT too
Remember you can fly IFR rules outside of controlled airspace even if you are climbing or descending for the whole portion so just file IFR for the whole route. Maybe they dont know what YZ means
or the computers are having a brain storm dealing with such complicated stuff so just IFR keep it simple
Pace