Knives over 3" blade length are already banned, if you can provide a valid reason why you have one over 3" you will be let off.

I was in my way to a camping ground with my camping kit in my rucksack and was pulled over and seached, they found my nice swedish steel 4" and was taken down the station for questioning. I was released with no charge as going camping was a legitimate reason to have a 4" blade. They even gave me a lift to the camping grounds after questioning!