They need more fibre.
There is no realistic way to just tap into fibre willy-nilly.
There is a tree-and-branch hierarchy to distributing fibre around a piece of territory, starting with a fibre spine, cables of up to 288 strands of fibre, and a bunch of "nodes" that house splice trays for jointing.
The nodes are the only point to "tap" into the network, using a tray to hold the splice.
Even then, the fibre itself plays multiple rolls. Fibre going to a cabinet is "point-to-point", running a gigabit connection over a strand of fibre and can't be shared. Residential fibre is a shared PON, grouping up to 32 properties ... and all strands of fibres need to run via a special splitter node before heading to the exchange. Any future FTTdp will need a separate strand of fibre that could be point-to-point, or could be a PON (we don't know yet).
Luckily, though, any cables laid down tend to have multiple fibre strands running between nodes. Tapping into the network in future will be a matter of heading far enough back into the network to find the nearest (and correct type of) node to add a splice to.
The worst case for you, though, is if BT are minimalist, and run a bare minimum of strands of fibre to the cabinet, with no nodes. Then there'd be a long haul to go back for a splice in the future.