Actually the KBAR was adopted because it was more of a utility knife than a fighting knife and as such was more"GI Proof."
I had heard this was one of those it is made in the US things.
The real fighting knives are of a dagger or stilletto design(prior to the KBARs adoption the Marine Corps issued mostly those designs including the 1917 Trench Knife, The Model 3 Trench Knife and the British Commando Dagger)
One quick question, have you got any evidence for the issuing of the British Commando Dagger to US Marines?
There are two schools of knife fighting use - Bowie, or Fairburn. It is a very old arguement that will go on until they perfect light sabers. A Ka bar is a bowie style blade so it is a fighting knife by default. The problem with these sort of arguements, is almost all utility blades can be used as weapons. I think it would be easier to list the ones that could not be used.
My understanding is that it has been banned from the UK. Look up KBAR.com for there full line of products, likewise for Ontario Knives.com.
Hennie has Ka Bars for sale so that would tend to say they legal in the UK. It might be worth a word over on British Blades, there is a thing about daggers so if Ka Bars have that Bowie edge sharpened this may become an academic conversation for a Brit.