I've lived in the UK (London) for a while now, but the longer I live here, the more confused I get about the dress etiquette for specific events...

For a bit of context, maybe unnecessary, I was born in Eastern Europe, where it's pretty normal to dress slightly more formally for weddings, baptisms, Christmas, someone's birthday, etc. Of course, it depends on the family, but generally, women tend to wear a bit of makeup (if they usually do) and dresses with heels or sandals for Summer, and men usually wear nicer shirts. Here? I see that happening less and less, and I'm starting to wonder if I'm committing a serious faux pas with my fashion choices. Some examples:

• I was invited by friends to Christmas dinner last December, and I wore a long black turtleneck dress -absolutely nothing outrageous- but then both hosts were wearing hoodies and tracksuit bottoms. I felt pretty stupid.
• I was invited by my boyfriend's family for Easter, which also doubled as a celebration for his father's birthday. I thought it would be a bit more formal considering the number of guests. Everyone (ten people total), except his mum and me, wore the type of 'house clothes' you'd wear when you want to be comfortable. Again, I felt a bit silly, like I was trying to draw attention to myself, even though I genuinely expected everyone would be at least smart-casual.
• Another Easter celebration, from my British side of the family, they were also wearing hoodies during Easter dinner.
• Two years ago, I attended two weddings where the dress code wasn't specified, and I swear, on both occasions, I had more makeup and jewellery on than the brides and their bridesmaids. Some men even wore polo shirts. As a plus-one who didn't know anyone, I felt ridiculous and people definitely stared, despite the fact that I always try to avoid looking tacky.

There have been many more examples, like going to live music events at nicer venues, where I'd assume people would want to look cute and dress up a bit. However, I often see people in hoodies and casual tops. I'll admit I'm a bit vain, I like my mascara and my heels - that's just how I was raised - but I simply don't feel comfortable always being the most overdressed person in the room. I've definitely toned it down over the years, for instance, I stopped doing my hair and I skip jewellery, but it seems like everyone else is being even more casual. I've spoken to my European friends about this, and they said it might not be a national issue but rather the type of circles I'm currently surrounded by.

For the record, I'm not well-off at all, I shop at charity shops and in second-hands. Most people I've mentioned here would be considered middle class.

So now I'm just really curious - is this entirely a British thing? Do Brits ever dress up? Am I misunderstanding social etiquette?