Yes, the show will go ahead regardless of the weather but in fact, it's forecast to be overcast and maybe a bit mizzly, but not chucking it down (= good weather in Cumbria
)
I've just come back from setting up. The show ground is already ankle deep mud in places, I'm sure the carparks will be by the morning.
It's a pain to get in, there's a one-car-wide bridge on the main route in and it can result in queuing onto the M6. Just gotta chill and queue. The road coming in the other way doesn't make a junction with the A590 though it looks like it might, so that's not much help.