the only legal EDC in the UK is a non-locking folding blade of less than 3". However if you have good reason to be carrying it then almost anything goes as Chris said. You just have to be able to justify it.

There isnt actually anything that it is illegal for a private citizen to POSSESS in the way of knives. There are some banned items autos, gravity knives, butterfly knives - which cannot be carried, sold, lent, given away, bought, imported etc. but possession isnt illegal. If you have one at home already it isnt actually illegal for you to possess it , you just cant leave home with it.

The other weird thing about UK knife laws is that it is illegal to advertise any knife based on its use in combat.