Yeah I was just being silly.
As your pics shows, flannel is a woollen material from which suits and trousers are made: think The Man in the Grey Flannel Suit
I think it was originally made in Flanders from English wool but I'm not sure about that. Anyway, when I were a lad we were taught that one wore a Tweed (or a Thornproof) jacket with flannel trousers: only Americans wore matching trousers. Hence the dumb joke with which I started.
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