Mumsnet is for the most part filled with champagne socialists, well to do and very privileged people with no real clue about majority beliefs. The majority here look down on the working classes and call them slurs for disagreeing with them, so don't expect much truth or fairness in these responses.
Of course he was being rude, it was on purpose. Real people accept that GB news often has interesting news, and that everyone should get out of their echo chambers and read news from all sources instead of just deliberately confirming their own biases.
But he was signalling to others that he's a good and loyal member of the party who NEVER engages in wrongthink or tries to figure things out for himself. He will do as he is told and he do his best to police your thoughts, opinions and ideas too.
When a person has no real argument they will try to silence the person who is making them uncomfortable or speaking up in ways that challenge their views. A good way to do this is to attack their character, or make them afraid to even speak at all.
What he intended to do was make you seem like a low class, horrible human for daring to listen to a news source his group deems wrongthink.
If he can "other" you he can then write any opinions off that may challenge the accepted slogans from his group.
It's quite deliberate, just watch others try to do it to me.