I don't think you understand the challenges of running an ISP.
Technically I suspect they could turn it now (or fairly soon), but it would be commercial suicide to do so. Having people on a trial is how they determine what problems are likely before rolling it out to the many thousands of "technically non-savvy" users who will simply phone the helpdesk who would be swamped. Apart from the pressures on the helpdesk, the bad PR would be very damaging.
They also need reliable and affordable end-user routers - something the CE manufacturers were (and still are) rather slow to come up with. It's no good having IPv6 if the routers you ship either don't support it, or worse still, support it but are unreliable (more helpdesk calls).
I do agree that they are a bit late to the party, but not half as late as others who I suspect still have no plans. But with a network the size of a national ISP, you don't just make changes without serious planning and testing. A small ISP specialising in tech-savvy users is in a better position, Plusnet doesn't have that luxury.
I dare say that they are working their way though the design, build, and testing of new management systems. The test accounts had to be manually created - real accounts will need to be automated, so that means changing every single system involved in managing accounts and the IP addresses connected to them. Not just trivial changes, but some significant changes. I suspect the next phase will be testing the new portal with a test group.
Even just adding an "enable IPv6" checkbox and defaulting it to off is a risk - many people will act in "if it's free, I want it" mode and turn it on, without knowing what it is or if their equipment will handle it. And what happens when there's a problem - yup, calls to the helpdesk and bad PR.
