I forgot to mention on here that there are pianos in the Divinity College. If you head to the basement, at least one of the classrooms (tutorial sized) down there has a piano. Go to the right of the building entrance, not the left, otherwise it may be locked.
A heads up though, the staff who work in there are really snooty. I went there during exams to find a good study spot (because the libraries are packed during exams) and someone who worked there came up to me and said "Can I help you?", and I said I was looking for a good quiet place to study, and asked if there were any classrooms or study spaces in the building that might be good for this. He looked at me and straight up said "No, sorry, I don't think we have anything like that here". And I'm thinking to myself WTF, it's a school for god sakes. Anyways, I ignored him, went down stairs, and within 30 seconds found an empty classroom. And he was the second person to approach me asking "if they could help me find something". Their students get to use the rest of our campus and many services, so we shouldn't be heckled about using their empty classrooms to study during regular building hours.
Lol, rant over. Just wanted to let you know - but just ignore their nosiness and play the piano to your hearts content. It's great because assuming there isn't a class going on, you get the room all to yourself.