It’s a tiny ass island yet whenever a British person hears another British person they’ll be like “Oi guvenor! I know exactly where in Merry-ol-England they are from! Clearly they’re from Bovinshire-upon-Weavilton!” And Bovinshire-upon-Weavilton is a town like 10 minutes away from where they live.
The US is probably the outlier here with too few regional accents given the vast land mass. I would attribute it to the population growing alongside the successive innovations of rail, radio and television so that regional dialects blended into each other.
And is probably why the east coast has more accent variation in a smaller area. New York even has accents that vary from borough to borough I am told!
Australia is worse some say we have 3 which isn’t true, but it’s certainly less than the USA.
media tends to lump “the south” into a single, monolithic accent that always seems to be some affected-as-hell Texas twang (where they pronounce “onion” like it has a “g” in it), but IRL there is a lot of variability between low country, piedmont, Mississippi delta, and southern Appalachian.
that doesn’t even get into who uses what idioms.
mass media has a way of flattening regional differences.