@JamesK:
Yes, the obvious distribution point for you is the garage to suit your existing LAN infrastructure.
As I see it you have two options:
Provide conduit for OR or their sub-contractors to easily run the fibre across the loft space from your gable wall entry point to the garage.
The flex conduit you suggest would do but a much cheaper option would be 20mm plastic (electrical) conduit such as this:
https://www.screwfix.com/p/tower-round-upvc-white-conduit-20mm-x-3m/6282pEasily joined to suit length:
https://www.screwfix.com/p/tower-white-heavy-conduit-couplings-20mm-2-pack/63776With lots of other fittings cheaply available for "going around corners", clipping to wood work etc.
Would be much easier to thread a pull cord, one length at a time before coupling.
If you decide to go this route, would suggest if at all possible that you drill through the gable wall and provide a water proof entry point to the conduit for the fibre to be inserted/secured to the pull cord, I'am sure the installing engineer would be more than willing, indeed delighted to make use of this installation.
Or alternatively:
Let the installer do his own thing and mount the ONT at a suitable location near the drop point (inside) the gable wall.
You pre-run a new ethernet (WAN) cable from the proposed ONT mounting point to the garage LAN distribution point or where ever you decide is the best Wi-Fi location for the router.
As you are running the new WAN cable across the loft space yourself I would not really see any requirement for conduit unless you really wished to do so.
If you have the necessary tooling and ability you could provide a fibre entry point on the gable wall preferably with plastic conduit, near a power point where you would prefer the ONT to be fitted.
The installer will then likely fit the CSP box over the entry hole you have provided, makes a neater job.