We had everything moved over to Fibre, so I terminated my copper line in the BT80 Junction Box
and connected up the supplied cable from the ONT to the NTE5A Master Socket which I moved closer to the ONT.
The main reason to moving everything to fibre was due to our copper line was that bad the line use to go dead when it was raining heavy, we also use to hear very loud screeching even when the Modem / Router was removed.
BT could never find the fault (so they say), they knew it was there so I was never charged when an engineer was sent out.
So moving it all over to new fibre and starting out fresh was out best option for us.
Granted the BBU might not last that long in a blackout, i.e. a few hours if that (not really had to try it yet), but we have packs and packs of rechargeable batteries to put in if the old ones go flat, so we are fine there.
What you really need to think about is would you benefit moving your phone line from copper to fibre (FVA), if not then don't, we benefited which was why we did.
Word of warning, if you move your phone line over to FVA you get a one off termination charge when you cease your copper line, I think ours was £35, hard to decode the stuff they put in our bill at the time (loads of refunds, discount's, compos, charges etc.), not too sure if they still do this charge so you might need to check before hand.
So its up to you.