Confused Newcomer
You're in Scotland, I'm in Australia. I know nothing of your rules and procedures.
However, here in Aus, the AIP directs one to change to 1013.2 AT or AFTER passing the Transition Altitude.
I'd be guessing that if you've set 1013.2 and then call "passing FLxxx" you're giving him/her a FL when s/he wants an altitude, thus s/he can't verify the accuracy of your altimeter or his/her radar readout of same.