echo that there isn't any need for joining the NCT. I did, but that's because I want to do their BF counsellor course, and you have to be a member to be able to do it. Can't say that the magazines are all that awe inspiring tbh, and as others have said, you don't need to be a member to go to any of the postnatal groups or bumps and babies.
On the other hand, they are a charity and rely on membership and donations to be able to provide the services, and their BF counselling service is really very good, so if you can afford it, join, but don't sweat it of you can't. I'm not even sure I'll be renewing my membership next year as money will be very tight by then .
Have a good time at Bluewater. It looks enormous on the TV, but I've never been.