The Criminal Justice Act (1988) says that you may carry a knife with a blade length of 3.0" or less so long as it is capable of folding.
Anyone considering using a pen knife in self defence is a bit silly anyway tbh...
run away
or
kick em in the nuts, then run away
are both acceptable options.
If you really really want to carry a weapon here are two useful ones that probably won't get you in trouble:
A powerful LED torch - some of these will temporarily blind/disorientate an attacker for a second or two at night time - gives you a bit of extra time to kick them in the nuts before you run away....
Some torches will also make quite handy kubotans - carry an actual kubotan around and you can be charged - carry a torch around - its just a torch. All the same - tis better to just kick them in the nuts and run away...
Other potential implement you could use is an umbrella - big, sturdy traditional style one with a point at the end - its just an umbrella so fine to carry but used in the same way you might use a rifle fitted with a bayonet and you can put some considerable space between yourself and an attacker.
Top tip though- don't bring an umbrella to a knife fight...
also - running away is almost always going to be your best option.