It's illegal to be drunk anywhere other than. in your own home and nit even then if there are small DC and no other suitable person in a fit state.
It's an offence to impersonate police and military any time, not just Halloween (so don't include rank slides or numbers, and make your fancy dress a bit OTT - it's the same idea as not carrying replica weapons, so make sure your cowboy gun is plasticy and shiny and could not possibly be mistaken for the real thing)
The stamp thing is an urban myth. Yes it's widely considered naff/disrespectful, but it's nit an actual offence.
My favourite is the suspicious salmon (rooted in anti-patching laws, I think?)