to "Go off
like a Cumbrian Taxi Driver" is another way of saying "to Go apeshit"
The phrase originates from the 2010 case where taxi driver Derrik Bird went on a murderous rampage through Cumbria, killing twelve innocent
people and injuring a further eleven. Apparently, the cause for his psychotic behaviour was a relatively
minor annoyance - caused by drivers taking fares from the wrong end of the taxi lane.
Typically speaking, "going off
like a Cumbrian taxi driver" is to overreact to a relatively minor annoyance, normally with extreme anger.