I'm afraid that there is no simple answer to this. It used to be that 4" folders (including lock-backs) were legal. Unfortunatly that has changed so that any edged or pointed object that is "deemed" to be an offensive weapon is illigal.
This means that although one police officer out in the countryside might see a fully serated Military as a camping tool (unlikely but possible), whereas another police officer might see a Ladybug as an offensive weapon.
Lugage is checked on a random bases if you go through the nothing to declare route. If I was coming back into the country with something "questionable" I would go through the red "items to declare" route and declare my copious amounts of alcahol for tax purposes, that way there is a lot less chance of them searching your lugage.
Although it is legal to by a Chinook in this country, I would seriously think twice about bringing one through customs.