Charity shop return policy

I purchased a shirt from a local charity shop yesterday, took it home and tried it on, but it didn't fit.

I took it back for a refund and was only allowed to receive a credit note. I was told this is because they have a changing room (which wasn't advertised and I didn't see) so you have to try clothing on there and then. I was also told they only accept returns on clothing, not on other items.

Not intending to bash the shop but I am interested to hear whether this policy stands re the law as I've not come across this type of policy before.