What the OP describes is a problem that occurs infrequently ... Due to some unfathomable issue the lines copper routing held on record is incorrect, so incorrect it is routed (on records) off a different cab.
The punter orders, the FTTC ties are allocated and provisioned to the cab correct by records, but this is of no use to the punter 'cos their line actually goes through a different cab.
Another fly in the ointment is contractors doing cab only FTTC provision tasks ... They don't give a monkeys if the quoted telephone line ain't present in the cab, just com(plete) the job and take the money. Now Openreach mark as complete, and the error doesn't come to light until a fault is raised.
Had one like this in Woodley, 5 times we went out, informed whoever that the ties had been provided in the wrong cab, and still it didn't get connected. .......
The OP will have to push and push again with his CP.
He is seeing ADSL speeds because that's what he has.... ADSL