I've noticed this in a lot of pubs not just JDW. The bar staff pull about 3/4 of a pint, take the money, top up the glass then run off before you can complain about a short measure. I might refuse to pay until I get the (FULL) pint in front of me next time I feel like some fun at the bar staff's expense.
Other fun things are to reply "yes fine thanks but I'd prefer a beer" when asked "are you all right there?" or give them a suitably short payment for a short measure saying "yes but that's for a pint isn't it?" when they question it.