There are quite a few people here saying that they 'would never carry a knife illegally'. Remember that it is not entirely up to us to determine whether that is the case or not.
If a member of the public believe that you are carrying an offensive weapon and your reason to do so doesn't satisfy the courts, then it's illegal.
I might feel that carrying a lock knife with the intention of doing some bushcraft whilst walking my dogs, is a fine and satisfactory reason. The courts may not.
Always double check your reasons and be certain that you are 'in the right'. I would never advocate making a reason up (doing so hurts our cause more than it helps it) but to have more than one reason is entirely plausable - so think before you carry