[alduin32]

I don't know how it works in the UK, but here in France there are many commercial ISPs that cannot provide IPv6 because the underlying infrastructure does not support it.

Yes, ideally, an infrastructure provider should not dictate what kind of IP packets the commercial ISP can use, and there are dozens of technologies that would allow them to provide transports that don't care about the routing layer, but in practice, these providers (that we never really hear about as end-users) often make very bizarre choices, for example setting up a complex DHCP interception schemes that feed both the OLT to "open the port" and a BGP route server to announce connected clients at the collection point, of course not supporting IPv6.

In one of the ISPs I work for, we literally have /48s assigned to all of our clients, but the infrastructure provider's doesn't route them to us : IPv6 traffic is rejected at both ends of the transport. The ticket has been open for years (IPv6 was supposed to be part of the network they were commissioned to build), and insiders told me that they had difficulties adapting their transport system for IPv6.

We may end up having to tunnel IPv6 through IPv4 to our clients, but we try to fight against that so that the infrastructure provider doesn't declare the problem "fixed" and decide that the proper way for ISPs to provide IPv6 is to tunnel it through IPv4 so that they don't have to change anything.