It all depends really. I'm not sure of the exact workings but many exchanges are 'daisy chained' together before hitting the big pipe. So your local exchange may link to a local exchange before finally hitting a big backhaul pipe.
When I was having issues Sky told me that my exchange was daisy chained to Kingston and that 7 exchanges were daisy chained to this exchange. Here was the large backhaul pipe for all of the exchanges. Obviously this means that a backhaul issue at Kingston would probably affect me here (around 15 miles away).