Earlier quoted context omitted.
I have read I can get it by upgrading to the very latest version of BT Home Hub (I am using 5). I ordered it yesterday. But my current Home Hub 5 is compatible, and I still don't have IPv6.
It looks like you would have got support on your existing device soon. A recent BT presentation says: "[IPv6 support] Home Hubs 4 and 5 support from early 2017" http://www.ipv6.org.uk/wp-content/uploads/2016/11/UKIPv6Coun...
Actually I just received my new router and I now have IPv6!