Count up all the minutes of early go's you get in a year (decent early go's circa 30 mins plus - lets not faff about) and I bet you get a hell of a lot more than half a day (4 hours).
What with getting away early on afternoons during winter and on spin shifts, I'd bet it easily amounted to a few days worth per year.
I know that it would be petty to keep people towards the end of the shift if they are not needed, but it is what we are contracted to do... All the early go's are dependant on the goodwill of management. Goodwill works both ways...
There are some things that NATS could improve on, but the local perks we get are better than you'd get in a real job.