[kemayo]

It's a slightly-tasteless joke resting on two facts:

1. The [King's / Queen's] Head is a common pub name in the UK.

2. Several queens have been executed by beheading at the Tower of London.

[azalemeth]

3. The word "head" in a nautical context also refers to the ship's toilet.

[Y-bar]

4. Beer head is also another name for the foam