No if you get gigafast Fibre to the property then there is no connection to the old telephone line at all. You can though (up to you to arrange) run an ethernet cable from the ONT to the router.
If you are actually getting Superfast 1 or 2, then it's FTTC (Fibre To The Cabinet) and the connection to your property is over the old fashioned phone line. In that case you can IF YOU DO IT PROPERLY have the modem router installed on an extension - but even done properly it will almost certainly have a detrimental effect on your connection speed to the internet!
