Check out this review on Amazon UK, which essentially backs up my findings too, kicking myself for having not read this before I ordered:
Most UK ISPs use PPPoE to authenticate users to their network, this is commonly used by DSL which if you have an OpenReach connection you will be using. Eero does not support PPPoE, which means you need to place it behind another router which leads to issues with NAT (the translation that a router carries out between public (Internet) and private (Home Network) addresses). This has stopped Sky Q from working in my house for On Demand, but I have seen vastly improved WiFi Speed and coverage in a congested WiFi area (3 story town house in city).
PPPoE support has an open request on the Eero support forums as a feature, but it has been open for over 2 years! Considering PPPoE and DSL are the two primary connectivity technologies in the UK, I am shocked that this has not been fixed before Eero was launched in the UK! My advice is, its not simple - if you’re a pro IT (which I am) then feel free to experiment, but for users looking at a simple plug in and go solution look at alternative Mesh solutions which support PPPoE and avoid the hassle of managing double NAT.