i can't really help you with the uk nozzle, but i had similar problems when i first try to use the adapter on my dutch lpg duster. it took me almost 15 minutes to fill up the tank. in the end the solution was simple for me. just hand tighten the adapter and then put the nozzle on. where i live (netherlands), you then have to turn the pump nozzle a half quarter turn (max!) (i turn clockwise so that the adapter stays screwed in) and then retract the handle to lock. fill up and than repeat the same process. taking the pump nozzle off is sometimes tedious, because if i turn it counter clockwise too much or too little, the pump nozzle doesn't detaches itself from the adapter (and sometimes the adapter starts unscrewing which isn't a problem but then my orientation of how much i have to turn it is gone so it takes a bit longer. need to screw it back in with the pump nozzle still attached first to get my orientation back.).
on my previous car i didn't had to use an adapter, so i had to put the adapter on and was able to turn the pump nozzle way more than a quarter turn to lock. same taking it off, i could use some force to find the sweet spot where the pump nozzle would disengage, because the filler nozzle was integrated in the car. on the duster with the screw on adapters i have to be more surgical.
in the end ==> hand tightening and only turning it half a quarter turn to lock was the solution for me and repeating the process backwards (max half a quarter turn to unlock!)
