I am an ex-BT engineer, and I had BT Infinity fitted in January 2014. The service has been really marvelous by the way, not that it wasn't OK before !
A new fibre cable has been laid to a new green Cabinet in your immediate locale. The new Cabinet will be sited very close to the existing one, but it has to have a electric supply fitted to it, unlike the old one. A link is the put in between the two Cabinets.
If you decide to take up BT Infinity, an BT Openreach engineer will visit your house and install a new router, and a new wall socket. He ( or she ! )
will connect you up in the new Cabinet, and your telephone line will run from your house, by the old copper or aluminum cable, to the old cabinet, be linked to the new cabinet and then back to the Exchange by the new fibre cable.
But the only bit you will see is the new router and wall socket ! You will have speeds of at least 38 mbs and maybe 76 mbs, depending on what package you choose.
Hope this is of some help, but ask again if you need any clarification !