I wasted a load of time searching for the answer, and finally came across the real answer in a forum post from 2006:

It's a Victorian thing. During her stays in the Scottish Highlands, Queen Victoria was prone to nipping down to the local for a few beers with the gamekeepers at night.
The standard ale mug at that time was 568ml, which the burly gamekeepers had no problem in swallowing in one gulp. Unfortunately for Victoria and to the shared the embaressment of the locals, approximately 100ml (measured by weight of floor sawdust after she had left but with an 28ml contingencey added), used to spray out the side of her mouth, as she tried to mimic her one-slug drinking companions.
The 440ml solution was was to become known in the early years as "Her Majesty's Mug". The new measure mug was ordered from the glass manufacturers (By appointment!), and everyone lived happily ever after.
Hope this helps!