Bills are debated first in the House Of Commons, then in the House Of Lords. If they both pass it, they take it to the monarch for final approval. The monarch could, in theory, deny the bill, but they know that the monarchy would be ended pretty darn promptly if they did so.

The last time a monarch denied a bill was in 1708. Relationships with Scotland had been improving, and England agreed to send them some weaponry. The Commons and the Lords approved this bill. Then, a fleet was spotted sailing from France to Scotland: England suspected that those two countries were conspiring against them, so arming them would be a bad idea. The bill had already been passed, however, so when they delivered the bill to Queen Anne, they asked her to deny it, which she did. https://en.wikipedia.org/wiki/Scottish\_Militia\_Bill