I know, I know... FAQ! I did look around the FAQ and /r/linguistics but I couldn't find an answer to the specific question I have in mind.
The title is something of a generalisation. For example, I know that accents in Australia tend to be divided according to distance from an urban centre (e.g. as a rule of thumb, the further from a city you are, the "broader" your accent).
Similarly, even people within London have their own accents.
One thing I can think of is that the United Kingdom is much older than (modern, Anglo-Saxon) Australia. Is it the case that time (and not distance) is the major factor in accents "splitting"? Are there any historical events which contributed to the particular divergence of accents?
ahaha! - I should really have added a section on languages/accents of Scotland to the FAQ by now. As always, there's room for more input and an expert overview, but meanwhile, here are some posts to get you started:
When did the majority of Scots and Irish people stop speaking Gaelic?
How different was the culture and language of the Scottish Highlands from that of the Lowlands?
Would medieval and Early Modern Scotland have predominantly spoken Gaelic?
What are the main factors behind the decline of the Scottish Gaelic language?
What seperated the Picts (northern Scotland) from the Britons (the rest of the island)?
... and here's a bit for Australia
and, if you don't get satisfactory answers here, you may want to xpost to /r/linguistics