This was posted on Head for Points this morning. I don't know whether it's appropriate in this case :-
If you paid for seat selection, you LOSE this money if you cancel. You may prefer to take the travel voucher as I believe the seat selection value is retained, either as part of the voucher or as a credit for free seat selection when you rebook.
Maybe ask BA if they'll give a voucher for future seat selection fees? I guess the answer would be no, but it's worth a shout.