First of all I'm not a lawyer but I have been able to find some case law which helps explain the issue about locking knives: http://groups.google.com/groups?hl=e....co.uk&rnum=16 Some other interesting stuff in the thread, particularly about knowing the law
Essentially the issue is twofold: firstly unless you have "good reason or lawful authority " in a public place you're only allowed to carry a folding pocket-knife with a blade less than three inches in length.
"Self defence" isn't a good reason to carry something outwith that description, nor is " I forgot", "I was using it earlier" but "I need it for work and am carrying it to/from work" might be. You'd have to be able to prove that though.
There's no specific reference to countryside or anything like that. It's up to you to show that you "have good reason or lawful authority" for carrying, eg a fixed blade knife, in relation to the activity you're carrying out, going to or coming from.
Now the lock knife bit, eg the leatherman wave, or Spydie Delica, or Opinels, is based on case law in which courts have found that the lock essentially stops the knife from folding and means that it is not a folding knife. Simple eh, debatable maybe, but that's what the judges have decided.
I suspect not getting yourself searched is a good start though.... I have a vague memory that some more recent public order legislation gives the police powers, in certain situations, eg demonstrations, football matches, to use a very wide defintion of what might be an offensive weapon so even your Classic SAK could get you in trouble
but remember I'm not a lawyer