Chocks,
if I remember rightly from my own retirement process, the right to retain your military title is spelled out in Queen's Regulations. It is the same for all three services. I have a feeling that it applies to all those of Captain (army) rank and above, not Major - but I may be wrong. As the modern general public seems not to value Service experience - and indeed, tends to regard ex-miltary officers as somewhat worthless snobs - it's very rare for anyone to use their military ranks after retirement, perhaps sadly.