Google a search for quarries. All roadstone (not monumental) quarries have a calibrated weighbridge. Give them a call to make sure that they are OK to carry out a 'public weighs', most are. It should be no more than £7-8 and they'll give you a ticket with a gross weight on it. A smaller operation may scribble your weight on a scrap of paper for free or a small 'drink'. It costs them nothing.