BTs name servers do provide query response for v6 records, hence peoples 9/10 on their "tests" and are fully "compatible with v6" whatever that means.
The "issue" is no public native v6 transport for the servers (I.e a publicly routed v6 address/anycast). Being in ISP DevOps myself I'd be 99.9% sure this has been done on purpose (Frankly it would be negligent not to be the case), to avoid a multitude of issues to the adverage consumer.
Those with clue can "roll their own solution" that works just fine. I imagine this will remain the case until such a time v6 deployments mature.
