I am far from being a BAA supporter, but I do have a little sympathy for them when it snows.
Because LHR is basically working at close to capacity, anything that reduces the number of aircraft movements [eg using a snow-plough] is bound to impact on flight schedules.
That is not to say that LHR should not have the right kit in place for when the weather turns chilly [!] but obviously whilst a runway is being swept, it cannot be operational.