[MovedGoalPosts]

You can't rely on a neighbour, even if they might be willing. What if they move, or fall out with you. Where would you be then. To ensure service from a neighbouring property you'd have to get an easement (legal right) set up. By the time you've paid the lawyers for all that, you might as well pay the BT £1500 fee.

Don't BT have some sort of legal obligation that they must connect a phone line if it's required. But I don't know if that places any controls on cost. You might need to hide that it is a holiday home, and make out it is a proper residence. Maybe a browse of somewhere like OFCOM?

All the other phone suppliers, talk talk, Sky etc, simply take over the BT phone line so it's that which needs to be installed regardless of who you eventually want to use. But it's because BT no longer have control over whether you migrate their phone line to somebody else, that they need to charge proper costs for installation since they may never recoup those costs elsewhere.

A mobile phone solution would seem the more cost effective, if that cost does have to be paid. After all, if you aren't there all the time, it would take a while to rack up that amount of charges?