FTTP is coming soon. I currently use FTTC and somewhere deep in a cupboard on my ground floor there is an ADSL splitter. The ADSL signal is carried over RJ45/Cat6 to the wiring cabinet on my first floor, where it connects to the Vodafone modem. Replacing this with a fibre is something I would not attempt. Moving the modem into the deep cupboard is also something I would not attempt. So my question is, how will the fibre signal get from the BT box to the new router (that isn't here right now) on my first floor. And who will do it. And who will pay ? And, by the way, do not mention wifi. If the wifi signal was strong enough to get through the house and annexe, they could hack it from Scotland (only a few 100 metres).
The router doesn't have to be adjacent to the ONT. Assuming the ONT will be close to where the current master socket is, you will need to replace the modem extension you are using with an Ethernet cable. However as you say you are already using a cat6 Ethernet cable as your modem extension you should be able to use the same one. (assuming it hasn't been modified, (i.e. had it's plugs cut off)
You will however need a mains power socket near the ONT to power it.
Most important thing is make sure you're there on the day and have a quick chat with the installer and ask him if he can do what you'd like. It's a whole new cable so really it's about ease of access and what's viable (as Jayach mentioned you'll need a 240v wall socket nearby so sort that before the installer arrives and make their life easier).
I've had Vodafone gigabit (950MBPS) and it was very exciting getting it in as the signal here was poor for 20+ years. When the guy was due (heavy snow and COVID so double the excitement) I had prepped an area exactly where I wanted it and he was happy to do that (I had it nowhere near the old VDSL setup and half way up the wall).
It's likely he'll want to follow the route externally of the old cable (assuming it's off a pole) so have a look yourself.
Tx for replies. Looks like the worst problem will be creating a nearby mains socket. The existing enet cable arrives at an RJ45 socket in the wiring cabinet, so connecting to the router should be easy. Tx for clarifying that in-premises cabling is wire, not fibre. However, does look like I'll have to have to check connection to the existing master socket. Will not be an easy task (I'm nearly 80 and not a small person).
They only care about power for the ONT. It's up to the householder how it's provided. If the customer wants an extension lead........
........... the customer is always right.
At the end of the day all an engineer is there to do is install the ONT and connect it with a fibre. They're not there to judge how someone wants to power that ONT.
Standards are well different to what we know from years ago.