I agree, you should
contact UK direct.
I noticed this stiffness a couple of times with my own UK Light Cannon, and before the last time, I had taken particular care to grease the threads a bit more liberally - but it still happened.
I believe this problem occurs as a result of air travel. The light is designed to withstand diving pressure, ie pressure outside being greater than inside the body of the light, then this pressure differential dropping again to zero on resurfacing after a dive. However it is not designed to deal with the reverse situation, where pressure inside its body is greater than outside.
When flying, the pressure outside the light drops to below that of its interior. Air then escapes via the switch mechanism, and equalises at ~0.75 bar (the typical cabin pressure of commercial airliners when at altitude).
When the aircraft subsequently descends, the switch seal shuts again, like a valve, sealing the light with 0.75 bar pressure inside while the outside pressure rises to 1 bar. The differential pressure of ~0.25 bar is sufficient to bind the threads on the head, making the head difficult to turn.
The answer, I believe, is to screw the head on only very loosely for air travel, so that air can freely enter and escape from the light's body during the flight.
Good luck with getting a replacement part.
