It moves. When I was at university in Reading I was the token northerners, being from Lincolnshire. I always thought somewhere around Darlington but now I’ve moved I think of Pitlochry as where the north starts.
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I forget who said it but “You aren’t a proper northerner until you call a Sheffield accent southern”
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seems to be accepted it’s around the Severn across to The Wash (if you’re in the south)?
Personally I’d say somewhere around the line from the Mersey across to the Humber.
So poor old Lincolnshire is in no-mans land again? Even I don’t know what we are, I don’t think of myself as northern but I’m definitely not a southerner!