At both Oulton Park and Snetterton there's nothing to stop you paying on the gate for a Saturday ticket, then driving straight to the camping area to 'view' the racing, camp there and you're already in for the Sunday. I know people who do that.
Even at Brands Hatch, they charge per person for the camping - there's nothing to stop you ejecting all your passengers before the camping gate, drive in & pay just for yourself camping, while your colleagues climb over the earlier barrier and meet you at your pitch. Trust me, I know people who do that as well
Equally, there's nothing stopping you at any of these circuits hiding some passengers in the back of your Bongo or just on the back seat of a car covered in coats, or in the boot. Oh yes, I know people who do that too. Only Silverstone seem to have it sewn up by making you walk through the gates.