Locked in a cupboard at work (Wales)

I work in a local cinema and a couple of days ago at work I entered a cleaning cupboard which is located directly opposite the inside of the entrance to a public bathroom. Whilst I was in the cupboard, i had the door open so it obstructed the bathroom door and a customer was about to leave so I pulled the door shut slightly so they could get passed. Except they push on the door as they're leaving and shut me in the cupboard.

The door has one of those number pad locks with a metal plate you twist to open the door, but on the inside of the door I turned the plate and nothing was happening and I could not open the door. I was banging on the door for 20 minutes before I was let out.

I had no way of calling for help as we are not allowed to have our phones on our person during working hours. And I wasn't given a radio either. To top it all off, my shift was 6 hours long, and I wasn't entitled to a rest break, however as I was stuck in there for 20 minutes, I ended up being 5 minutes late which puts me over the legal limit of 6 hours with no break.

My question is, has anything that happened to me in this incident illegal? I feel that a door that is unable to open from the inside poses some sort of health and safety hazard.