It's a brand-new BT router so I would be surprised if that was the case. I suspect the non IPv6 connectivity rather come from which node I connect to on the network.
If you're on VDSL try grabbing an HG612 from ebay and using that instead, with a decent router behind it. I was running a dual stack setup like that for over a year without issue, before moving over the Virgin Media and having to rely on a HE.net tunnel.