Don't worry if you're refunding a bmi 236 stock ticket. According to ba.com/bmirefund:
Your refund will be calculated in accordance with the fare rules of your ticket. A bank transfer or cheque (not necessarily your original form of payment) will be processed in due course. This may take up to 8 weeks.
They'll refund direct to your bank. It's probably easier this way for BA as bmi is not accepting new bookings. Although bmi still technically exists, Diamond Club is still operated by bmi (as it says on the copyright of diamondclub.org)