The truth is that as NATS don't isue the ticket they don't recogise it for future use. You might be able to persuade a civil non NATS airport to allow you to work free and this way you can get your tickets validated for airfield work. But this wont work at the larger NATS airfields such as Heathrow ETC. the Mil ATCO who work at Swanwick/Prestwick do so because the Goverment Via the Military says they can and therefore give them permission to do so. You'll find that any retired Mil ATCOs only work at training new ATC's not actually controlling.