Luke - I think that's a site for scouts - not the public but I'm not certain.
Wild camping is tricky in Poole. Most of the coast line of the harbour is either privately managed or owned. We have poached a few times just for one night at Arne, several kayaks come over from Lake road slip and spend a night here quietly but I thinks it's not really allowed as its RSPB managed here. The camp site at Redcliff is ok.
As for camping next to the beach Alderney is hard to beat, nestled behind a large earth bund to protect it from the wind and a safe harbour to moor up in its a great destination if you feel like an adventure.