Why are fifty pence pieces pentagons?

Answer 1

The British 50 Pence coin is "heptagonal" in shape, ie. seven sided. (If it were pentagonal, it would have five sides.)

The Royal Mint, as do many other mints around the world, produce some coins with different shapes as an aid to the vision impaired.

Answer 2

Most coins around the world are either circular or rounded polygons with odd number of sides. The main reason is that because the polygons are constructed by making arcs of equal radii from the opposite corner, the resulting face is equal in width in any orientation so that machines and coin sorters can always distinguish the values whether they are inserted corner or side first.