PANS ATM doesn't help a great deal.
4.5.2 Aircraft subject to ATC for part of flight
4.5.2.1 When a flight plan specifies that the initial
portion of a flight will be uncontrolled, and that the subsequent
portion of the flight will be subject to ATC, the aircraft shall
be advised to obtain its clearance from the ATC unit in whose
area controlled flight will be commenced.
4.5.2.2 When a flight plan specifies that the first portion
of a flight will be subject to ATC, and that the subsequent
portion will be uncontrolled, the aircraft shall normally be
cleared to the point at which the controlled flight terminates.
...
4.5.7.1.2 When prior coordination has been effected with
units under whose control the aircraft will subsequently come,
or if there is reasonable assurance that it can be effected a
reasonable time prior to their assumption of control, the
clearance limit shall be the destination aerodrome or, if not
practicable, an appropriate intermediate point, and coordination
shall be expedited so that a clearance to the destination
aerodrome may be issued as soon as possible
In principle, I guess 4.5.2.2 means a controller should be aware of a flight planned route leaving controlled airspace 6 FIRs downroute. In practice, 4.5.7.1.2 means that most controlled flights will be given a nominal clearance to destination, on the assumption that usual procedures for handing over traffic to neighbouring sectors will be used.
A foreign pilot files a flightplan, receives and ack with the plan unchanged and then on starup at LKPR gets told 'Cleared to EGHH via the TANGO1, departure 124.25 squawk 2654'
What would you say if you were told?
'Cleared to leave controlled airspace via the TANGO1, flight planned route, departure 124.25 squawk 2654'
It's technically correct!