The £15k seems a fairly sensible approach to me - a comparatively small outlay to keep a trained soldier/sailor/airman in for another few years. Of course, it does mean that those who were going to stay anyway will get the money as well, but that is no different to any of the other FRIs handed out in the past.
I have long given up any hope that anyone was going to show an interest in retaining someone in my rank/specialisation, and while I would love someone to throw a bonus my way, it is not going to happen. IMHO, there is a general acceptance that FRIs cannot compensate certain people when compared to opportunities outside/pressures of Service vs home life at key stages. They are now focussing on keeping people in at the early stages of a career, rather than fighting a losing battle with those that have been in 10-15 years and know their own minds.