Here is the protocol from the Queen of England's website -- which applies even to her own subjects:

"The Queen meets thousands of people each year in the UK and overseas. Before meeting Her Majesty, many people ask how they should behave. The simple answer is that there are no obligatory codes of behaviour - just courtesy.

However, many people wish to observe the traditional forms of greeting.

For men this is a neck bow (from the head only) whilst women do a small curtsy. Other people prefer simply to shake hands in the usual way.

On presentation to The Queen, the correct formal address is 'Your Majesty' and subsequently 'Ma'am'. "