It's largely irrelevant that 21CN runs IPv4 as it's implemented as MPLS VPN switching network. What the customer (i.e an ISP) gets presented with is basically a

It's largely irrelevant that 21CN runs IPv4 as it's implemented as MPLS VPN switching network. What the customer (i.e an ISP) gets presented with is basically an L2 ethernet connection, over which they can run whatever protocol they want, including IPv6. There are competent ISPs available that will utilise OpenReach and that do offer IPv6. BT Retail is not one of these. The real issues with 21CN are that many parts of the edge network have multiple SPOFs (so a single failure can result in multi-hour or longer downtime for customers served by multiple ISPs that connected to an exchange) as well as insufficient bandwidth provisioned for those links in some areas. Another issue, is that backhaul from exchanges is metered at a fairly expensive rate.