Out of interest why did you book it through Expedia rather than on BA.com? I am not saying BA will be helpful here, but Expedia will definitely be unhelpful.
24 hr rule does not apply if the flight is within 24 hours. So either you booked a refundable tix and you are good, or you have at least a changeable one, will change the date and make your next jaunt to London an expensive one, or otherwise you are £500 poorer but richer in experience!