duluth
Can't comment on the union thing, but as an ex-apprentice and an ex-manager hiring staff for UK Government, I can say you already have some good advice from the others-
Don't be late! Arrive clean, shaved, neat clothes, sensible hair. No need to go over the top with clothes, but clean shoes and nails are important. Go to bed early night before so you're awake and recharged. DON'T SMELL OF BEER!- so NO CHUG night before! Turn Mobile off. Be interested, concentrate on what's being said, because you must think on your feet and be ready for the surpises that happen at interviews. Have some sensible questions ready, like "what training will I get". "Why do you want to be a ******" is THE Standard Question, so have your answers thought out. Most important is to act polite, adult and reliable- no one wants to hire anyone who appears irresponsible or immature.
Oh, and Best of luck with the interview!.