I've never had a problem with the HG612 holding the line, but did wonder whether a more modern modem may be a good idea. So decided to try swapping the XyXel over. It seems fine so far, and stating a max attainable 6Mbps higher than the HG612 does. The HG says just under 60Mbps Max but often reports exactly 60000kbps sync, as per my sig.
The ZyXel also gives exactly 60000kbps sync but max 66000. That's going to be today's question to AA. Along with why their log reports a BT value of 57nnn kbps! They also apply their own line speed to that, like Plusnet do to the IP Priofile, but considerably lower than that BT-reported sync.
Re the static IP Address, you missed the question. Are you applying the address that contains the two :1111: entries? Or some other? My earlier IPv6 BQMs were with addresses that turned out to be LAN ones, not the WAN one. Though they worked for BQM, which was neat, despite being LAN not WAN.