I can say that last year I used ST alot for the family and the total, when reckoned using full fare tickets, was in excess of £50000. However, that must be tempered with the reality that you might not get on and if the flight is full it becomes a ST lottery!! First class returns to SFO, HND, EZE, PVG and SIN. All confirmed, all on time and well looked after. Not once bumped.
However, 99% of the downroute hotels welcome family guests and offer fantastic discounts to BA staff when travelling on holiday and the route network encompasses most major cities world wide. Growing as we speak.
All told, not a bad benefit.