I finally have IPv6 support from my mobile provider - TalkHome Mobile (UK)!
I woke up this morning to see a new 'VoLTE' icon on my phone. Checked the status and see an IPv6 address on the mobile interface for the first time, and it works. Woohoo!

On the downside, I currently cannot receive SMS messages and the provider's app is broken. Hopefully this is just over-running maintenance.

#IPv6

TalkHome use the EE mobile network, EE's APN and therefore the BT ASN 2856. They are very much a no-frills provider, I'm paying £2.89 per month for a 3GB SIM-only deal.