I came across this 2010 BT presentation on
"Light Grid" access networks
It is obviously a product of BT's research side, rather than Openreach's practical side.
However, it covers the concept of connecting 2 adjacent exchanges with an over-specified fibre spine - allowing for the dual-parenting of cabinets (so moving the resilience of duplication from the exchange level down to the cabinet level). It then increases that to the concept of routing the spine between 2 core nodes (in 21CN) via multiple exchanges (and also mentions the concept of bypassing exchanges in the future).
As we now know that FTTP-on-demand is likely to have been enabled by over-provisioning the fibre spines, it seems plausible that BT have employed this kind of setup.
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