Hi,
Moving to other exchange, there are now a few options available:
CNI (Caller number Intercept) - This is an announcement in BBT exchange advising caller of your new number. Fixed quarterly fees.
RCF (Remote call forwarding) - This seamlessely forwards all calls to your new number without caller knowing. You will still be charged a line rental+RCF charges+call charges (from your old number to new number).
BT VoIP - You can have BT VoIP on your business broadband and get the number ported on broadband and get all calls forwarded to your new number.
I dont know the size of your business but if you are a big business, you can go for Lease line with SIP in which case BT can port all your existing numbers on SiP and you can take your numbers wherever you go!
Hope this helps 
