Another tip for the NEC that I figured out myself last year was that there are different prices for the car park if you pay at the automated machines at the exhibition!
When I went to pay, the machine asks which show you visited. I initially chose the motorcycle show but my inner geek got the better of me and I cancelled and tried again. The food show was the same price as the bike show. Then I tried the Scale Model show, and that was cheaper (I think it was £12 instead of £16) so I paid that instead.
The ticket barrier doesn't know which show you visited, and doesn't care as long as you have a paid ticket.