Only 24% of London bus drivers decided to vote in the dispute in 2014, yet there were two one-day strikes. The 2014 strikes over ticket office closures were triggered by a ballot that attracted only 40% interest and in which only 30% of the relevant workforce voted yes.
To those who say that we politicians have no cause to set thresholds, let me remind you that in America, the land of the free, 39 states have banned strikes by mass transit workers.