Not sure how you don't get it, you're in a new flat, nobody has ever paid for the line to be installed, BT have basically been allowed to install it so that a future resident can use it. You are therefore paying for an installation that has already been done. No future residents of that flat will ever have to pay it (as long as it's not moved away from BT, even if it is like the OP then I think it's more arguable).
The OP's house has a physical BT line, that's where the similarity ends, it has previously been an active BT line so someone has already paid the initial installation cost.
The fact that BT sees it as the same does not mean you're in the same situation, you could get discount by sky paying it or agreeing a longer contract with BT but it will get paid. The OP will be paying a connection fee someone has already paid for.