You can have the BT ONT installed up to 30 metres from where the fibre termination box lies.
Basically the connection between the ONT/external fibre box is a pre manufactured fibre optical cable. My engineer told me they come in two different lengths (one being 30m). Normally people ask for them next to their master sockets, but some people have asked for them in other areas. There are some limitations as the cable is not as easy to feed as normal telecoms wire.
Apparently BT have to use the pre manufactured fibre connector cables because the tubing/fibre cable they run (externally in the ducts) does not like bends and would get stuck or break if they were to run it inside peoples' properties.