So I ask myself is it a coincidence that there are builders on the site and broadband went off for a wide postode area a week ago
Yes, it is a coincidence.
Openreach doesn't own the telephone exchanges any more; it sold them off years ago and leases them back. So in the long run, it does want to exit the leases.
But in the short term, simply turning off service to a bunch of users would not gain them anything. On the contrary, most of those users with failed connections will be claiming a daily penalty for the loss of service from their ISP, and those ISPs will be claiming it back from Openreach.
Openreach won't be able to exit exchange buildings until:
1. All voice telephone exchanges are turned off. This has been put back to Jan 2027.
2. All ISPs have removed their LLU equipment from the exchange (i.e. ADSL and voice equipment which connects directly to the copper)
3. Any other services hosted at the exchange, such as leased lines, have been migrated away.
Because of the complexity of all this, the initial trial of ~100 exchanges was scaled back to 3-5. Even if you're in the next 100 it's still going to be a while.