The trouble with a plant room, is usually it's a mess of odd sized "things" connected by lots of pipes and cables, which even if done neatly, is still some industrial eyesore to most people. So no I would not have that in a WC.
But in our case we are having a combined utility room and WC. I thought BR might have something to say, but they are happy with it. all they want is an accessible toilet and space to fit a shower in the future. They don't care if it also has a washing machine and a tumble dryer in the same room, and they don't care if instead of a wash hand basin I have a kitchen type sink in a kitchen base unit instead.