I moved my FTTP to Aquiss a couple of weeks ago, and am very happy. In short, I'd take 300M from Aquiss over 1G with a larger consumer provider any day.
The 300M FTTP service is £45/month with no setup fee, but the first six months is half price, the contract is 12 months, and there's no automatic price rise at end of contract term.
(I was on Cerberus before, who were also pretty good, but the same speed is now £51/month with them, and there's a £48 setup fee)
You're right that it's Entanet's backbone; their address blocks show as "Cityfibre Limited" in he.bgp.net (Cityfibre
bought Entanet in 2017).
I can't fault Aquiss' customer service. I raised a ticket before the service went live to request an IPv6 allocation, and it was allocated almost immediately, by Martin himself. It's a proper ticket system with a portal - no need to pick up the phone at all.
When the service was live, I requested IPv6 reverse DNS delegation but there was a problem with Entanet's nameservers. They escalated it to Entanet and it was resolved in a few days.
You get static addresses for both IPv4 and IPv6.
Bear in mind that you don't get a free router, but for me that's a plus as I already have my home network set up the way I want it. On switchover day it was just a case of changing the PPPoE username/password and renumbering my IPv6 networks.