I think people are losing sight of the importance of context.....
I personally would find it distasteful to see/hear someone noisily slurping soup from a spoon in the context of a normal meal.
However, if I visit a ramen restaurant in Japan or in the UK I appreciate that "slurping" is a traditional part of the eating experience - designed to allow you to lift the noodles with your chopsticks whilst both cooling and allowing you to slurp up the rich broth that is core to the dish. Similar with Chinese soup dumplings where you slurp out the liquid contents of the dumpling.
I'm equally not going to break out the silver service in a Chinese restaurant, I'll adopt the cultural custom of chopsticks or in the case of a burger or crayfish in the US - I'm going to eat it with my hands (juices dripping down my arm).
Good manners are as much about context as anything else.
However the original point of this thread was focused around UK manners around the table in public and private.
For me, yes they are important. I want my children to be comfortable in as many environments as possible and to know that what's appropriate in some settings is not in others - but fundamentally that process starts (for me) with the basics of UK good etiquette (holding cutlery properly, mouth closed, no elbows, no gadgets, waiting for everyone to start, asking to leave the table, saying please/thank you etc etc).