The Trade Union Act 2016 requires a vote for industrial action to have:-
a 50% turnout from those eligible to vote; and,
if the strike will affect “important public services”, then at least 40% of those eligible must vote in favour.
The Act is not yet in force but commentary suggests that the new laws will come into force on the 1 March 2017.
The Southern Rail and London Underground strikes would fall within these definitions, but the British Airways cabin crew strike would not.