A Crown Court case (Harris v DPP), ruled (case law). A lock knife for all legal purposes, is the same as a fixed blade knife. A folding pocket knife must be readily foldable at all times. If it has a mechanism that prevents folding, it's a lock knife (or for legal purposes, a fixed blade) The Court of Appeal (REGINA - v - DESMOND GARCIA DEEGAN 1998) upheld the Harris ruling stating that "folding was held to mean non-locking". No leave to appeal was granted.
http://www.goxplore.net/guides/Knife_law_(UK)