I guess they don't want to break network segmentation.
I can certainly see why they'd want the new fibre network to be segmented effectively, but that doesn't mean that it has to be a direct 1:1 overlay of the copper segmentation.
Certainly there is no need for the new fibre installation to *have* to go via the cabinet, and there is no need for the network topology to have to look identical.
For example: I don't know what the deployment plan looks like for the chain of aggregation nodes as they snake out from an exchange - but one document I saw from BT suggested that they should add the extra fibre, to change the topology from a tree/branch structure rooted at a single exchange into double-ended chains that ran from exchange to exchange, giving backup routing to the core network.
Either way, there is certainly no need to assume that there is a 1:1 correlation of aggregation node to PCP.
However, I am perfectly prepared to believe that the restriction is in place for ease of administration