Surprisingly even in the great British weather I rarely have a class where it rains the whole time but if there is rain we continue with the class and I find shelter with trees, you would be surprised how much shelter a large tree can provide.  Make sure you bring your rain covers for the buggies and you will need a raincoat.  The only reason I cancel a class is if there is a thunderstorm or if there is too much ice so it becomes unsafe underfoot but I will let you know by text/email prior to the class.