?, the dealer told you 40%, if the legal limit is 25%, no need to test it they told you, also looking at your pictures then yes its obviously too dark, i was actually sticking up for you as they really shouldn't have sold you it like that but hey ho !!
oh and you added the plate yourself didnt you ?, its a private one ?, not the one that came with the car, when purchasing a plate you get more than a few warnings about spacings etc, i got a verbal warning from the police about my spacing, i knew it was wrong and got away with it for a few years, but when caught i have now gone legal with it
It's your responsibility to ensure your car is legal.
Ignorance, "everyone else gets away with it" and "well the someone told me...." are no defence in court. Sorry.
I agree it was harsh getting a fine, most will ask you to rip the tint and prove you've removed it a few days later. The plate maybe should have had an advisory too, but the law is the law. It's like walking around with a kick me sign on your back and then complaining when someone boots you up the arse.
oh i suppose you would have new that the tint was 10% more than should be even if you had bought from a dealer not everone knows a great deal a bout cars and you sound like one of them