419: an oversimplified answer is that the Gatwick staff accepted changes to their T&Cs a couple of years ago – the alternative was the closure of the BA base there. BASSA did not fight very hard on their behalf.
The imposition of changes that the strike is supposed to be about broadly brings manning levels at LHR down to where they already are at Gatwick.
Despite being the ‘poor relation’, the LGW people do seem to be considerably more well-adjusted than their LHR brethren (and their customers seem to appreciate it too).