big hit
yes, the pins are supposed to be in the cockpit...but mistakes do happen.
now, I don't know for a FACT that was the reason the left gear didn't retract...there might have been another reason, or multiple reasons.
there is an old joke stateside about an arrogant delta pilot followed by another airline...arrogance abounded by the delta, and the following less arrogant pilot finally mentioned that since DELTA LOVED TO FLY AND IT SHOWS...it might be wise to remove the gear pin before it showed how stupid they were.
amazingly enough, if you takeoff and can't retract the gear and simultaneously lose an engine...you have real problems.
one might call the CAA people at gatwick to find out the whole story.