I really don't understand Danny's problem. There is no law banning the purchase of the knife he's after.
Here's a UK online shop selling it:
http://www.heinnie.com/Knives/Ka-Bar/c-1-92-155/There is a law making it illegal to market a knife as being suitable for combat, but it just applies to the marketing, not the sale.
As to carrying knives, the law states that anything "made or adapted" for causing injury, or intended to cause injury, can't be carried without reason. However, most knives aren't made or adapted for causing injury, they are tools, and as such it rests on intent. If you are carrying a knife around a town centre without a good reason the prosecution will claim intent to use it, if you are out on a camping trip then the intent is obviously to use it as a tool.
However, I don't ever remeber studying something called Secton 21 of the Knives Act 1997, which specifically precludes carrying of certain blades, restricting them to military use only,
I think you are misreading the act. It's about marketing of knives. It's illegal to market a knife based on its suitability for combat, unless you are marketing it to a military force.