You made your mark on the register and 'took the King's shilling': a coin symbolically given and taken as your acceptance of being in the service of the Crown.
The British army
You had to be 18 to enlist for the British Army in World war II, though some boys who were younger were able to enlist using false documentation.
By the British in World War.
The same as every other army - they fought.
'Jerries' was British army slang during the second world war for Germans. In the first world war, British soldiers were known as 'Tommies'.
They were soundly beaten by he German Army.
Not in high numbers, no. Most "defectors" Into Axis ranks were soldiers from British colonized lands.
Propaganda mostly.
Red Coats
About a 160,000.
Dunkirk
Yes, it was invented by a colonel of the British Army.