Could I ask exactly what happened with the cyclist? Never having taken a bike on a Pendolino, I'm not familiar with the secenario. Where was he that he needed a staff member with carriage key to let him out? I presume this is a sort of guards van for bikes – but how then did he get in? Are the staff supposed to unlock the door then wait to relock it, and somehow it shut while he was in there? Or what?
On a Pendolino the bikes go in the van area next to the Standard class (Coach A) end drivers' cab. This is accessed from outside through a door which requires a carriage key to open and close, but can also be accessed from Coach A via a sliding door which while it says Staff Only or somesuch on it, is not ever locked.
What happened was that the cyclist waited for a short while by the door, then realised nobody was coming. Then he tried to manhandle his bike through Coach A to the passenger door, not an easy or quick thing to do without clouting people as the aisle is narrow - a road bike would fit, but a straight handlebar bike is very awkward. By the time he reached the passenger door, it had already closed and locked. So, seeing this, I pulled the handle to prevent departure and get the issue dealt with.
I wouldn't have done it normally, he'd just have had to go to Coventry and back, probably no massive hardship. The problem here was that there were diversions on, and the train I was on (which was terminating at MKC for only LM stopping buses that nominally did not carry bikes) was the last one to connect with the 15-car Voyager "Blockade Buster" via the Chiltern line on which he wanted to travel to London. So had he been unable to alight, he would have been left with a 50 mile cycle ride from MK to London or a need to abandon his bike at MKC.
I think the need for the key is sort-of to enforce mandatory bike reservations - but it is often forgotten about by staff even when the passenger correctly has one.