and here is a more stupid thread on knives:
http://www.theanswerbank.co.uk/Law/Question1278467.html
The law has significantly changed since my fave
Offensive Weapons Act 1936
carrying knives to be sharpened
and butchers carrying their stuff (in the boot not under the seat) too and fro were covered and clearly lawful
I am not sure if there is still the clear division between obviously unlawful - chair leg studded with nails
and could be lawful - butcher carrying stock in trade (the knives not human carcases) or swiss army knife on a nature trek.
I would worry about carrying a swiss army knife to a football match.