A British pint is larger than an American pint - 20 as opposed to 16 fluid ounces. A British fluid ounce, however, is about 4% smaller than an American fluid ounce. A British fluid ounce of water actually weighs an ounce.

Because of the weights and measures act of (1985), the only things you can buy by the pint in Britain now are milk and beer. The weights and measures act mandated that items must always be priced by their metric weight/volume. The act made exceptions for some goods that were traditionally sold in certain sizes - draught beer and milk being the most notable (Imagine nipping out for a litre...)1.

In the interest of having a single standard, the British pint is now defined as a metric measurement, equivalent to 568 ml.

1 - However, due to the ever increasing influence of the European free market, it's becoming more and more common to see beer sold in 330, 440 and 500 ml bottles and cans.