I would like to make one observation - Lydd to the Isles of Scilly at FL50 is not IFR. It's completely the wrong quadrantal, it should be FL40 or 60
Not if the flight is under ATS control i.e. a radar control service, then any level is OK. I routinely fly at FL100, 110, 120, 130 .... 180 all in the same direction.
It would be very wrong for the airfield of departure (or whoever you used to file the plan) not to address an IFR plan to the two IFPS (Eurocontrol) addresses.
That doesn't bear much relation to reality, in the UK.
If you are IFR off airways in the UK it is probably better than the FPL is addressed as if it it is VFR.
That's what normally happens, in the rare cases where somebody flying within the UK on the IMC Rating files an IFR flight plan.
Addressing it to the two IFPS addresses is correct but that will not faciltate crossing clearance of any Class D you may find along the route.
I wonder why? Surely any flight plan sent to Eurocontrol goes to all the sector controllers? What exactly is it that achieves the implicit enroute clearance?
Neither is sending it manually to any en-route airfields. Few, if any, will take any action upon receipt, but would rather wait for the 'free call' when you call.
That I can see allright.