Have they managed to put IPv6 on all their broadband gateways yet?
When I was experiencing the infamous "single thread performance" issue (see thinkbroadband), and after my complaints were being taken seriously and not just dismissed, the first thing they made me do was go back to v4 only, as they had only enabled one broadband gateway for v6. Proper ISP indeed. This wasn't that long ago - 2016 I think.
Meanwhile, on "not proper ISP" BT, IPv6 just works, didn't have to ask for it to be turned on, don't need to care about which part of the network my router is load balanced onto - and I don't have single thread performance issues!