I presume that the silly sod had left his phone in his jacket?
You can legally carry a Leatherman Charge, TTi etc) in the U.K. provided that you have lawful excuse to do so. Religion, sport, tool of work etc.
You can carry about your person at at all times something like a Leatherman Juice XE6 which does not have locking blades without having to provide lawful excuse.
The female part of the lock tends to be external and is held in place by two screws. That is what I would go for first. Failing that I would whittle or saw a section out of the jamb.
Just as an aside: I had to extract a college's little girl from a bathroom. The "rescue" slot on the outside has its uses.
Mind you, I also perpetrated a small wind-up on my colleges by going round and using my leatherman to close and lock every stall in the toilets.....