Originally Posted by
drhandsome
By the way you can also do this at the gate if you forgot to do it before getting past security and you didn't check in any bags or you're connecting. Happened to me a few times.
When I have had this message I wasn't able to enter security until going to the visa check desk.
Once, I got this message when connecting from CX to BA (HKG-LHR-EU) and CX had even given me a boarding pass for the BA flight. The boarding pass wouldn't work at the entrance to security.