This is a question fer you Limey's. What IS "plain flour"?

I'm looking at a hot water crust recipe from Paul Hollywood, one of
the "masters" of the Great Brit Baking Show. It says "450g plain
flour" plus "100g bread flour". I get "bread flour" and thought
"plain flour" might be what we call all-purpose flour (APF). So, I go
to the link provided and get a BBC page that explains "plain flour" is
soft flour. Sounds like it might be pastry flour (or Wht Lilly or Martha
Wht) flour. Yes? No?

I looked at the link fer "white bread" flour and it comes up sounding like
APF. Yes? No?

So I guess the question is: what is "plain" flour and what is "white
bread" flour?

nb