Typically, 'Bridge Mode' refers to bridging two devices together via a WiFi connection. So your ISP box would send out WiFi and your Opal would take that WiFi signal and turn it back into a wired network like a 'backwards' AP. You can connect them via ethernet cable, but either way, Bridge Mode disables all the routing logic and other features and basically turns your Opal into a basic ethernet switch. I'm guessing there must be more to it than that, otherwise why use a router for such a simple task?
Again, I am not familiar with UK ISP's. But if your ISP box is still doing the routing then what I think you may want is the Drop-In Gateway feature instead.