I hate to say it but the 'Queen is above the law'. In her role as sovereign she is immune from prosecution. On top of this, in English caselaw (can't recall the case off the top of my head), it was held by a former Lord Chief Justice that the monarch could not be required to give evidence in the courtroom.
Prosectution is brought in the name of the monarch and so she can also intervene. A typical example of this is when the theft trial against Paul Burrell collapsed. No doubt plenty of lawyers with Republican tendencies would like to argue that in the advent of the HRA98, things have changed. The chances of success?... What do you think?
I don't pretend to be unbiased by the way. I am a proud Republican and am unreservedly opposed to the monarcy, the whitest of white elephants.