Re: British RPF most scary DR WHO monster?
Two things.
1.) I'm breaking the rules because I'm American and posting here anyway.
2.) I only started watching Dr. Who as an adult.
That said, the Weeping Angels in Blink are ABSOLUTELY TERRIFYING. The Autons are pretty freaky, too, mostly because they work on the same concept: they're everyday items that you see, which normally seem innocuous, but in Dr. Who turn out to be deadly.
I think the episode The Impossible Planet would've been freaky, too, but more because the monster is the Devil himself (well, sort of).
Come to think of it, if I'd been around 6 years old, a LOT of the monsters from Pertwee's era and on would've scared me. The Green Death, as I recall, had the miners all melting and turning green and such. That was pretty freaky, too.