Had their Betternet1000 (900/900) product installed Tuesday, switched over to my existing pfsense setup yesterday with no issues. Was told during installation that I would need to phone customer service but didn't entirely buy that at the time, and didn't need to - simply switching the WAN setup from PPPoe to DHCP (after switching connection from the modem to the ONT) took care of it.
I did add the static IP as I have some services which I expect cgnat will mess up and didn't want to have to spend time troubleshooting.
Pretty solid so far:
It replaces gfast with EE which I will be terminating. I am getting the full 350/50 service, but seem to be getting more and more disconnections after 18 months of it being rock solid. With the impending price increases it works out pretty much the same for the remainder for a far faster service. I was in contract until middle next year but terminating the contract was ~£330.