Found a fantastic location for a meet today...
Bowood house, wiltshire. Their web-site
It's about 10 mins off the M4... (near Calne, Wilts)
They have a huge (and i mean HUGE) parking area (which is free), and surrounded by fantastic scenary. A coffee shop next to the car park...
And of course, you can pay to enter the house & gardens, which also has a adventure area for the kiddies...