Do make sure you can actually get connected! I had the unhappy experience of having to wait well over 6 months for a connection, even though I had a BT line from the house back to the distribution box. Seems they had run out of lines from the box back to the exchange and decided to use the pair (unused) that went to my house. Nothing was thrown up when I placed the order but on the day of the connection, a BT engineer arrived, after about 20 mins of doing things made the unhappy announcement that there wasn't any spare lines and couldn't even say when that might be.

That would be an example when costs could be significantly higher. That potentially means they would have to run new cables back to the exchange which if ducts have failed could be an expensive proposition (and even if the ducts haven't just getting 2 guys out to pull new cables through ducting over distance is likely to cost a lot more than £50).

But, is it a new customers fault that the BT line plant in place is at full capacity on the E side or would it be down to BT ?
Also if they decided to increase the number of avaiable pairs by installing a new cable bundle which wouldn't be only a single pair, they over time would re coop more than whatever it costs to install it, just as they have with the rest of their copper line plant,

But that is why BT flatten the costs across customers. In the end BT are a profit making company therefore can't throw many at new lines without somehow recovering it. All of the costs for supplying are ultimately paid for by end users - should you penalise one user for £1000's because it needs a new cable run? Or flatten that across users so someone with no work required pays the same as someone who needs new cables run?

Thanks for the insights all, when you put it like this it seems good value for money! Since the cost of an install is regulated, does that also mean they are obligated to provide a service? In the event they'll never recover the cost of installation in wholesale revenue, a property might never receive a landline.

Hypothetically, what does that mean for the ISP/telco i.e. Plusnet? Would they then release their customer from contract?

BT have a universal service obligation to provide a landline with the caveat that it must be at a "reasonable" cost. I believe this is normally interpreted as installation work equivalent to £3,400. Beyond that the extra is typically born by the customer. So if you live three miles from the nearest line, then expect a big bill.

However, it should be emphasised that this limit will most definitely not apply to any cost involved in upgrading cables to concentration points (so from the exchange to a PCP or more localised connections). That will be viewed by the regulator as a capacity planning exercise on what is a shared bit of the infrastructure. Indeed the network planning should avoid getting into a position where all the available pairs are used up. It will, of course, happen and that has the potential to cause delays of many months. However, a shortage of pairs should not, in itself, be a bar to getting a line installed. It just might take a while.