Assalamu Alaykum Ustadh, my cousin claims that since we live in the UK, criticising Saudi rulers isn't imperssible since they aren't actually our rulers. I know this is an incorrect statement, but what can I advise him?

So if his father is not mine I can publicly criticize him and mention him in that which he hates which is gheebah? No. May Allah preserve our rulers and governments and give them all success and goodness. This is the aqeedah of the people of sunnah and jama'ah is to make dua for the rulers not incite people against them.