It is but the employer doesn't have to there is lots of confusion on this topic, I've not had to deal with a new mother in the last few years so I might be rusty but......
There is no direct right to breastfeed/express at work. But it is considered best practice to help people who want to express do so.
New mothers ... should be given time off, a private room with a lock and refrigerators to store the milk.
Now those are recommendations, the wording is should not must....so you couldn't for example demand that work gets a fridge if none exists at the moment or demand they get you an office with a lock if there isn't one already.
There could be an argument that under H&S laws you must be given a fridge as its your employers duty to protect you in which case they could remove the risk by banning expressing for example. So I would always advise people to ask never demand in situations like this, most if not all employers will say yes unless they are particularly bad.
Sounds like your boss knows what to do, maybe a "DO NOT DISTURB" sign would work as well as a lock esp if the office is small.
I await to be corrected and I'm not HR expert I just implement what they tell me but that's how it was last time I checked.