I think the danger is the anti rave laws.
It doesn't matter if your not if the police think you are its all over.
....Unfortunately that is the reality.
I don't know the 'anti rave laws', nor when a private gathering is deemed to be either illegal or an illegal rave. This is how the big all-night clubs were born - The raves had to be taken indoors where they could be more controlled.
A really big public car park would surely be ok to meet and start off with and have photography. And then, as the member with Hoo Hoo Dillies suggests, a
private field somewhere for camping etc.
But even a field owner might quite reasonably insist that no open fires were built directly on the ground. We used to take dustbin lids (metal of course!). Dustbin lid raid on the locals, anyone?
I know a really big village cricket field in Dorset (a flat and very large field which very amateur cricket is played on) and the owners are used to lots of cars and tents on it too. But it's way down south in the UK and so too far for many folks.