FWIW SIRO ran fibre into my estate a couple of years ago. Houses are all semi-d so it may be different to how they're handing terraced houses, but in our case they run the fibre along existing electricity poles on the road and only request access to your house if you order a connection from one of their providers. At that point somebody would come back out and string a fibre from your house to the nearest electricity pole and connect you into the system.
It seems plausible that for terraced houses where they cannot run fibre to each house individually they're getting ahead by having permission in-place for when somebody else orders. While it's possible it could be a scam, they should only need access to the outside of the house at this stage so it should be obvious if they start asking for permission to root through your sock drawer that something is up.
FWIW I'd agree with the others that the 1000Mb/s speeds offered by SIRO/Eir fibre is fantastic and extremely reliable, so I'd take their arm off to have it available to your house!