Update on today's install. Great OR engineer.
Pros and cons discussed. On their suggestion we brought the external fibre into the adjacent room (which has power) through a new hole and located the ONT in a sheltered spot. So I'll have to do a Cat 6 run to the "server cupboard" but it's actually already a shorter/neater run then what was there before, and avoids the downsides of an in-house fibre run.
They were very happy to remove the existing copper and MS. I'll just need to do a tiny dab of mortar outside and fix a blanking plate inside and we're good.
Had a bit of "fun" locating the end of the external ducting with my SDS that some previous bodger owner had buried under a slab and brick step. But OR was happy for me to do this while they did other bits of the install so saved a rescheduling.
Router temporarily hooked up to ONT in the kitchen, connection speeds amazing!
Thanks for all the advice