"Over the centuries, knighthood has evolved: it is no longer awarded solely for military merit, it cannot be bought and it carries no military obligations to the Sovereign."
Quoted from Her Majesty's Website.
It's not necessarily all correct though, a former New South Wales Premier, Sir Robert Askin was rumoured to have paid the Governor of the time (also a knight) AU£10,000 (AU$20,000) for his gong, no doubt without Her Majesty's knowledge. Still, it was just a rumour.