Many employers require their employees to follow a dress code, but not necessarily a dress if you're a man! I have never seen a male cleaner wearing a dress. Have you? In this company are there other male cleaners, have you seen them wearing a dress? Is she saying to wear the dress over your trousers?
I think that maybe employers are allowed to enforce different dress code standards for men and women. However, they may not impose a greater burden on either gender. And in this case it would be a burden for you to wear something that is alien to you. So go find out your rights, phone citizens advice and ask about employment law, and be armed with the correct information when you accept or reject the job.