T&C 15.2. If flight bookings for Award travel on British Airways or on flights operated by an Airline Partner are cancelled by the Member within 24 hours of departure, Mileage in respect of the Award will not be re-credited to the member. In the event you are prevented from travelling by an Event Beyond Your Control we will refund your Mileage provided you have an unused Award, have informed British Airways promptly, provided evidence of the event if requested to do so and paid the applicable fee specified on ba.com.

So no miles refund unless you can prove "Event Beyond Your Control".