This question was being posed to them a decade ago when I was a customer with them, and their position on IPv6 still remains one of mystery today. They will not disclose much (if anything) regarding their plans or any ETA. So I wouldn't get your hopes up.
I plan to move back to Zen next month though. The IPv6 issue can be easily solved with a tunnel but obviously native (what I have right now) is better. Unfortunately, it's becoming a bit unworkable being on a metered connection and Zen are the only other quality ISP I know of (and have been with previously) where the connection seems stable, reliable, fast and unmetered.
Zen have no plans to provide IPV6, although they say (and have been doing so for years) that they are testing it.
There is a thread on the Zen forum, and the latest official statement from Zen says " However, in short - there are no plans in the near-term to provide IPv6 support for customers on our Broadband services"
BT (poor) -> Zen (excellent) -> O2 (started well, went downhill -> IDNet (No complaints - but 100GB cap) -> Zen (unlimited)