need an outdoor venue in london to hire in August?

we are looking for a venue to hire in August, preferably in South London, but will look at the whole of London as well. We will provide marquees/equipment, ect so just need a space and electricity/generator. Wondering if anyone knows of anywhere or has been to a wedding in a outdoor venue? I have pretty much no leads at the moment so any kind of suggestion would be appriciated.

Thanks