Thanks Russ for the link to the Metropolitan Police site -- very informative.
It is illegal to ... carry a knife in public without good reason - unless it’s a knife with a folding blade 3 inches long (7.62 cm) or less, e.g. a Swiss Army knife (a "lock knife" does not come into the category of "folding pocket knife" because it is not immediately foldable at all times) ...
Redacted.