I’ve heard from a Brit of my acquaintance that it’s because gingers are stereotypes of Scots and of course, the Irish.
Read what Brits had to say about the Irish (and the Scots) a couple of hundred years ago....you’d think you were reading Nazi propaganda about “subhumans” and “inferior races”.
Apparently, the prejudice is still there, but now the underlying reason for the prejudice has been forgotten; now it’s just a prejudice against folks with reddish hair.
As late as the 1950's in the US northeast it was not uncommon for businesses to put out 'Help Wanted' signs with the word 'NINA' on them. NINA stood for 'No Irish Need Apply'.