an advertisement that said "100% of this area can now receive broadband, except for those individual lines which, for one or more reasons usually associated with the deterioration of the copper line conductor, cannot." wouldn't have much impact, would it?
BT's slowness in getting broadband to outlying areas (or even some not so outlying) is simply related to cost. They have to invest considerable sums in exchange upgrades, and they're a Limited company - they have to justify their actions in terms of profit to their shareholders (who are also their customers). Sometimes it isn't financially viable, and those exchanges have to wait until enough people register their interest to reach the trigger level that's been set. If enough people never register there's a problem, and that's where BT and the government need to get together to come up with alternatives.
Line quality is often the problem, and in those cases BT can do something - they can renew the line. The problem there is one of fairness - if they renew yours I'll want mine renewed too, and so will that man down the road, and his son, and so on. There must be millions of dodgy copper line lengths in Britain, we have the oldest public phone network in the world.
All these problems will be solved, in time, but the investment required to bring broadband to every corner of the nation is truly huge, and shareholders like receiving dividends - they don't like to be told there's no money for them this year, or next, because of the cost of renewing the network. When shareholders get that kind of news they pick up the phone and sell their shares, and when that happens it doesn't take long before the price plummets, and then the company's in big trouble.
Non of which is of much solace to someone in rural Staffordshire who can't get broadband because he/she is 100 metres outside the distance limit. Or someone who is inside the limit but whose line doesn't pass the ADSL test.