In the UK, it's unlawful to have any kind of offensive weapon in public, unless you have a reasonable excuse.
Self-defence isn't an excuse you can use, the courts won't accept that, unless in exceptional circumstances where your life or someone else is in serious danger.
You can carry on you in public a folding, non-locking pocket knife with a cutting blade no longer than 3 inches. You can't use it as a weapon though.
Most weapons specifically designed for killing are banned from the UK completely, such as the push dagger or a zombie knife.