Yes, I admit, as an AmE speaker, that all non-North American accents sound the same: BrE, Irish, Scottish, Australian and South African. Or rather, I can tell they are different if placed side by side as in the excellent 21 accents but I can’t name them off in isolation, unless I pick out one very specific feature (e.g. he said ‘bairn’ for ‘baby’, must be Scottish, she said ‘bruvvah’ for ‘brother’, must be London).
Is there some way to say objectively, comparing them all to each other, that, say, dialects A and B sound more similar than A and C or B and C? And really, with specific dialects…To Australians, for example, does AmE and, say, BrE actually sound alike?
I see a distance matrix:BrE AmE IrE ... BrE 0 7 2 ... AmE 0 4 ... IrE 0 ... ... ...
(of course the off diagonal numbers are picked out of thin air). Is there some less subjective comparison of distances (like how many people of one kind misunderstand those of another)?
Which ones are the most ‘out there’? Do the accents in the British Isles share lots of similarities but the former colonies have little with each other or the isles?
Have a look at this highly relevant paper:
Towards an automated classification of Englishes
by Søren Wichmann and Matthias Urba
from the Max Planck Institute for Evolutionary Anthropology (Leipzig)
Have e.g. a look at the tree-like structure on page 4, Fig. 3.1.
(unfortunately the referenced paper therein doesn’t seem to be available publicly).