My gut reaction was that you’re wrong and it’s easy but actually thinking more about it, even in a small area like my side of London there are a lot of what could be considered high streets, even with different parts of the same street acting as different separate high streets for different communities. A more concrete example is Uxbridge road near Shepherd’s Bush and the section near Shepherd’s Bush Market being two distinct high streets, and then a couple k’s down the road it turns into Acton High Street (Acton itself having multiple distinct high streets). It’d definitely be a challenge to map something like this if you’re not familiar with the area.