For what it's worth, I have NEVER seen BT terminate the SWA on their cables. It appears to be there for mechanical protection, not earthing.
In every new build I have seen the BT man come, they strip back the armour a LONG way. In your case they would strip back the armour to under the floor, and leave just the inner cable coming up through the floor.
Then you just have a 10 pair cable going into your master socket and 1 pair is used.
They normally leave all the other pairs just folded up an stuffed into the back of the box, presumably in case they are needed in the future.
If you have some slack under the floor, that's what I would do rather than extending the cable.