Thanks for clarifying - so the £27.75 figure is a net credit showing on your account.
Of course, if you have any accrued service usage since your most recent bill, then that might explain why BT are not giving you a refund for an account credit; I guess they will want to be able to offset it against the bill. Your refund will only be on any balance remaining thereafter.
But do you have your own estimate of how much you think that final bill is likely to be? Do you think it's likely to be less than your credit balance, or more than it?
If you think it's likely to be less, then I can see why you might be annoyed by BT dragging their heels about generating the last bill.
Bills are normally raised on a regular basis - I think mine are every three months but maybe some customers get monthly bills. Assuming that you got your bills every three months while your service was live too, has it been three months yet since your most recent bill?
Forgive me but I don't really know what rights consumers have in the event that they have a credit balance on some utility or other service they are no longer using, to be offset against a final bill that they haven't yet received. However, I imagine you'd be in a stronger position if BT have left it later than usual to send you your bill.
