As I don't live in Britain, I cannot know whether Mrs Thatcher, Brexit or Covid 19 has been responsible for the changes there, but I can say, that much the same thing has happened here in Denmark since I moved here in 1970.
At that time, it was usual for strangers to converse in buses or trains and usual that people who lived in the same street or in the same block of flats knew each other by name and greeted each other when they passed on the stairs or on the pavement. If you had time, you chatted for a bit, if not, you just said good morning or evening according to the time of day.
When you moved into a flat, a neighbour would knock on the door, introduce her-or himself and say "If you need any help, just knock on our door, would you like a cup of coffee`"
We kept an eye on each other's children when they were playing, and when my parents came to visit a neighbour meeting them said, "Oh, you must be grandtante's parents, nice to meet you."
In schools pupils were supposed (and did) say good morning to every adult they passed on their way to their classrooms before school started at 8 a.m. the child wished the adult good morning, then the adult returned the greeting, using the child's name if we taught her or him, or otherwise knew the child's name.
In some firms you shook hands with your colleagues every morning and you certainly wished them good morning, and good evening every day.
In the 1980s and 90s all that changed and now we are seeing the kind of society were a lot of people, perhaps the majority, never offer to help anyone, don't bother to introduce themselves to new neighbours or colleagues, and where a vast majority of parents and teachers apparently believe it unnecessary to teach children good manners or consideration.
So, this change seems a sadly international phenomen and greatly pre-dates Brexit or Covid-19 pandemic - if anything the pandemic made quite a number of people here wonder if life might not be just a bit more enjoyable if we revived conversing in the street, offering help etc.