Why do pint and half pint glasses in pubs have a crown and a number on them, and why does the number

Why do pint and half pint glasses in pubs have a crown and a number on them, and why does the number alter? Also, would anyone be interested in starting up a 'glass spotters' club? And why can't we sell bananas in lbs but we can sell beer in pints?

Paul Trussler, London, UK