Re the plug-in extension, that is normally bad news on ADSLx, and potentially disastrous on FTTC as VDSL2 is far more sensitive than ADSLx. For a start, nearly all contain a ring wire, which can cost up to 2.5Mbps downstream sync. If there are also wired-in extensions in the house, they will also have ring wires unless you have dealt with them.
Take a look at this page
, and also the Miscellaneous Nasties.
I believe Zen will be supplying the separate, (engineer installed) Openreach modem. You can order a data extension kit, (Openreach call it a Home Wiring Solution) which was certainly free for a few years and I think still is. If interested, you need to check with Zen. The standard version of that is an up to 30 metre run from the master socket, to provide a VDSL2 socket for the modem wherever you want. The main drawback is you have to put up with what may be a cable running where you don't want to see one.
This does have the disadvantage of adding some copper to your distance from the FTTC cabinet, but the effect should be small.
Instead, you could neatly pre-install some unconnected (good quality as some cheapo versions aren't twisted pair) CAT 5 from the master to the desired location, leaving plenty of spare at each end, and ask the engineer to use that for the purpose. Still order the kit as that allows for free relocation of the socket for the modem, and most (real Openreach) engineers will move the master instead if you wish, using that cable.
That sort of setup should, but no guarantee, give you the higher end of the "Clean" estimate. If is was a self-install and you used your existing wiring I would expect towards the bottom of the "Impacted" range.
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