Interesting question. According the Wikipedia:

All persons enlisting in the British Army and the Royal Marines are required by the Army Act 1955 to attest to the following oath or equivalent affirmation:

I... swear by Almighty God that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth II, Her Heirs and Successors, and that I will, as in duty bound, honestly and faithfully defend Her Majesty, Her Heirs and Successors, in Person, Crown and Dignity against all enemies, and will observe and obey all orders of Her Majesty, Her Heirs and Successors, and of the generals and officers set over me.