For those who are British: What are the absolute must-know cultural quirks?

I'm planning a long-term move to the UK and I've read all the guidebooks, but they really only cover the obvious stuff like queuing and tea. I feel like there's a whole layer of unspoken cultural rules that people just 'know' about, and I want to avoid making any major blunders. For example, is it true that you never talk about the weather unless it's raining? Also, what's the deal with regional slang that isn't just 'brummie' or 'scouse'? Any advice on the subtle social codes or things that are just generally considered 'very British' that I should be aware of would be massively appreciated.