I suppose CG NAT or IPv4 tunneling would be acceptable, but only when dual stacked with a dedicated IPv6 address. Just to bundle hundreds or thousands of users on one IPv4 address without also giving them an option of a unique IPv6 address would be horrible.
This is a classic case where the regulators should come in to encourage or enforce the preferred option (IPv6) instead of a fudge (CG NAT).
And logging will be fun, the next time the likes of Ben Dover start fingering an IP address, hundreds or thousands of people could be using it at any one time. I wonder if Plusnet have considered this scenario for the trial?