I get what you're saying, and I'm not suggesting you are fixated with stereotypes. But we are talking about the difference between not European celtic/saxon/scandinavian etc people not between north european and south european latin peoples were there is an obvious difference because of african eastern invasions and counter invasion. I would be hard pressed to tell the difference between an irishman an englishman a scot and a welshman and if you look at the genepool in this region its a right mixup of all 4 (invading soldiers don't keep there dicks in there kilts for long). So french man and english man decent chance of spotting the difference but within the brit isles way too hard.No, I don't mean stereotypes - although there probably is a certain amount of statistical basis to some of the stereotyping we associate with various groups. I'd be surprised if there wasn't a statistical correlation between being Welsh & having blue eyes, I'd be surprised if the average height of a the Welsh was anything close to that of Norwegians - although I've never seen figures. But a stereotype is just that - a crude & insensitive characterization. I'll give you an example of what I mean.
I buy petrol in a garage over the road. Loads of English people pass during the summer on their way to the Llyn Peninsula. There's this lady who takes money for the petrol & you've often got a huge que there. I've often stood queuing & noted how she virtually always starts the conversation in the language appropriate to the customer.
She might be able to spot cultural or class characteristics (most of the local customers would be working class), she might have a good & recognize most of the locals by sight - but she'd need a good memory, it's a very, very busy garage.