If they cannot achieve the required clearance for an FTTP drop,then how are they achieving it with your phone line?
They may not be.
When the minimum height restrictions came into force (I think as part of the NRSWA in the early 90's), it didn't apply retrospectively, but only to new/adjusted installations.
So if the dropwire is below 5.5m and was erected before the requirement became law, that's OK.
If it needs to be replaced or a new dropwire is needed, this has to meet the height requirements.
They can replace the pole with one taller than standard - that is quite common and a relatively simple task.
As for dual use poles - why can they not continue to use it? They is less danger with fibre than with copper.
The existance of overhead power is a complication, but not necessarily insurmountable.
Where BT shares use of power poles, the BT wires will be below the power cable with a minimum (c. 1m) gap between the two. If this means the point where BT can attach the dropwire to the pole is too low to meet the road height retrictions, another solution would be needed.
It's not impossible that BT could install their own (taller) pole, but this would likely mean that the BT dropwire would cross over the top of the power cables - which isn't usual practice.