I’m not even British but I’ve read enough novels to know of those things.

Morriss dancing isnt a real thing. It is just something silly invented by the BBC as a self-deprecating joke, like Canadians street vendors selling fried beaver tails.

I've been to this, it's great:

https://strawbear.org.uk/

It's a load of different morriss dancing troops dancing around the town with a parade with the lot of them. And there are a lot. Bloody freezing some years though!

Also, in the same vein, this weekend I think there's the annual Sea Shanty festival in my home town.

https://harwichshantyfestival.co.uk/

The videos are quite bad at conveying how great some of the groups are, especially sitting in a smallish pub with a pint listening to some amazing singers do sea shanties. Quite magical.

What is a pooh stick and is it related to a poop knife

You and your friends get a stick each, together you drop it off a bridge over a stream (upstream side), then you run to the other side of the bridge, whichever stick appears first is the winner.

We have conkers in London.