the costs of exiting these sites is a modest one-off cost
The metallic pair rearrangement is hardly modest from the information made public (Multi £m), but would still be cost effective if the rent went up enough. Once the existing lease is finished the landlord can basically raise the rent to astronomical levels enough to force any tenant out over time.
The issue with the OP is that they are with an ISP that doesn't provide FTTC and therefore was/is not interested in any offer OR have made. His way out is to leave and take a FTTC service from any ISP that provides one.
This is the same issue that affects all those people complaining about slow BB that will not upgrade from ADSL to FTTC/P in other parts of the country. Better services are available to many BUT people won't change either due to cost or cussedness.
The disproportionate effects do fall on ADSL customers BUT that is why OR have made the offers of FREE for x months rental to migrating customers. This is also part of the cost of exiting the building.