Ok so I have looked at the ZyXel modems and can see what has been mentioned already that they are expensive at £140+. This is a lot more than I was factoring in spending.
Not sure which ZyXEL models or where you have been looking but yes sound a lot more expensive than I would have thought?
From a modem point of view, was thinking more along the line of XyXEL VMG1312-B10A, 8324/8924-B10A, all are modem/routers but have a good reputation in modem service with their internal front end filtering, yes, they are all old models now but still giving good VDSL-2 service to a lot of forum patrons and can still be aquired at very reasonable cost by shopping around.
Believe the ZyXEL VMG3925-B10B modem/router does not have the internal line filter.
Would also avoid the VMG1312-B10D model, budget Broadcom chip set, tried on my line, very poor results.
Can't comment on the original OR issue ECI modems have never used, I'am sure others can advise.
Have used an unlocked Huawei HG-612 modem which worked really well on my line and provided full DSLStats access, quite a few are still using these on their lines, appear to work very well with Huawei DSLSM's, would not write them off just yet and they are certainly very cheap and readily available.
Once you decide on a modem then the router is very much your choice, I get the impression that your current preference is a 2 box setup?
I also read about the BT HH having a hidden menu for 'Bridge' mode but alas it doesn't appear to be in the Plusnet hub one unit.
Have a PN Hub One, which is a re-badged BT HH 5A as far as I remember, was terrible on my line, can't advise on bridge mode configuration, would never wish to use.
With regards to the ECI modem, do you just connect it to the phone line and then run the LAN1 cable to the WAN port on the router and then have all the connection settings in the router or do you have to log in somehow to the modem?
Can't directly advise on the ECI modem bridge setup but would think it's just the same as any bridge configuration.
With my VMG1312/3925 modem/router setup I use an additional ethernet connection from the modem to LAN to access the modem GUI, obviously with different modem/router IP address allocations.
A RPi 3B running DSLStats is also ethernet attached to a router port for 24/7 stat's harvesting.
Cross post from "meritez" while compiling acknowledged.