Just FYI there is a much easier way of getting to the car park without having to go the wrong way and without the speed bumps round the back.
When you come off at the Bluewater turn off, keep in the right lane, carry on straight over two roundabouts, get into the left lane and the road forks off to the left (follow the signs to the "Upper Plaza" yellow car park). You're then on the top level of the car park, keep going forwards until you are forced to turn right, and head to the part of the car park furthest away from the centre itself.
It's been really busy recently so I'd recommend meeting at the first part of the top car park you get to going this route (when you get onto the roof, first right). This gives us oodles more space and fewer cars around for better photos etc.