Best example is likely the dismissal of Australian Prime Minister Gough Whitlam by the Governer General: https://en.wikipedia.org/wiki/1975_Australian_constitutional...

My understanding is that royal assent does not need to be granted, but that in practice there has to be clear evidence that granting assent would lead to a disaster. It's kind of an emergency protocol. I highly doubt the brexit would qualify.

Interesting quote from Wikipedia: In 1914, George V did take legal advice on withholding royal assent from the Government of Ireland Bill, a highly contentious piece of legislation that the Liberal government intended to push through parliament by means of the Parliament Act 1911. The King decided that he should not withhold assent without "convincing evidence that it would avert a national disaster, or at least have a tranquillizing effect on the distracting conditions of the time".