I think the very existence of the monarchy acts as a deterrent to things like, for instance, what we saw on 6th January 2021 in America. Also, technically the military have allegiance to the monarch, and the police the government, which means another division of power that makes it far more difficult for dictatorship to arise. It's not perfect, but I definitely prefer it to the American system.

But as to the specifics of your question, whilst royal assent refusal hasn't happened, I think there has been unofficial pushback on occasion. The Queen's rumoured resistance of Margaret Thatcher's stance on apartheid South Africa, for instance (Thatcher was slow to condemn or impose sanctions and so forth).

I do think the presence of the monarchy acts as a deterrent to dictators, although that said, the Tories have certainly had a good go at it a number of times. That's one reason why we need this current lot gone. I have no political allegiance, and view politicians essentially the way I view nappies, ie a functional necessity that need to be changed regularly, or they smell, as they are invariably power crazed, often psychopathic, and quite frankly none of them should be in charge for too long.