I have seen in previous posts that it takes 5 working days when account is closed to receive any credit refund, currently been waiting since bill on 19th for a £700 refund
try contacting billing using message now and they should be able to help
https://www.bt.com/help/contact-bt/account-and-billing/broadband
That seems rather a large refund, are you a BT Business user?
I moved to EE and this is the credit on account plus refund of advance billing
Have you asked for advice on the EE forum?
It still seems a large amount, as normally it would just be a month`s advance bill.
Got through via phone. Even though it said on bill they would refund, someone had to request it. Reason so high it's a quarterly bill and it was start of billing cycle.