Moving from BT to Virgin, question about moving back

I'm pondering moving from fibre on a BT line to Virgin Media (cable) taking my current phone number with me.

If, in 12 months time, I want to move back to a BT based provider (i.e. Sky, Plusnet, BT itself etc.), would I have to pay the £140 charge for a new line or would it be connected again for free? I'm not planning on changing the wiring or socket or anything like that.