Staff travel isn't taxable as it does not cost the employer anything to supply,
Staff Travel DOES cost BA something. The main cost would be the additional fuel burnt, and there would also be the cost of meals and other consumables used. Perhaps someone who knows can say whether there is a net gain or loss to BA from ST.
My guess would be that at the back of the plane the 10% of normal fare paid does not cover the additional cost, but that in premium cabins it probably does (because the extra fuel burn is the same where ever they sit on the plane (excluding additional luggage allowance)).