About the weather- We have family in Scotland, and they are usually wearing thin(ish) waterproof jackets with a jumper and a shirt in very late August. They haven't resorted to waterproof trousers quite yet when I see them (v. late August), so by early September, they proably haven't got them out yet. I'd say a Scot would be in full wet-weather regalia by late September, but some years I'm sure they've had to bring out the umbrellas earlier.
About 'haven't the foggiest'- I use it sarcastically, and so do most people I know. I don't know of anyone who uses it in the proper sense, but I'm sure there are.