-- slicked back hair is a term that describes hair that's been coated in gel or pomade or something then combed straight back with a tight comb giving a slick appearance.
-- punk spiky hair... I'm sorry.. this isn't self explanatory already?
-- the Union Jack, I believe is what they call the flag of the UK, just like the States' is "Old Glory"
-- "retro" is a term which describes dressing or acting in a way reminiscent of how they used to "back in the day..." it could be 60s, 70s, 80s, 90s, it doesn't matter... it's a really loose term and means anything that's cool for being "old school" as opposed to being cool for being contemporary or modern.