In the UK, the law is fairly tight. I am not going to go too in depth about it, but my source of knowledge is here
if you want to check up on it, or get some examples. In general:
You may not carry a knife with more than 3" of cutting edge, and it may not lock open. Butterfly knives, flick knives, or other assisted opening knives are also banned; if you are caught with one in public, you will end up in jail.
Knives over this length may not be carried without good reason. The exception to this is :
1. Knives carried for work
2. Knives carried for religious reasons
3. Knives carried as part of national dress
As for what a public place is:
The Prevention of Crime Act 1953 section 1(4) provides that;
'a public place includes any highway and any other premises to which the public have or are permitted to have access, whether on payment or otherwise'
This also includes your car (even if it is locked).