Hi there GT,
What you need to do is call the approach/radar controller at an airfield (civil or military) close to your departure point. For example, taking off from Wellesbourne(EGBW) for an airways flight a pilot could call Birmingham(EGBB) Approach once airbourne. The EGBB APP controller would be in contact with the airways controller and could get an Airways Joining Clearance.
The main thing to remember in this situation is to STAY OUT OF CAS UNTIL YOU HAVE BEEN CLEARED TO ENTER.
Happy flying...
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Pull back and the houses get smaller - pull back a bit more and the houses get bigger!