Don't always believe Wiki Red:
http://www.blades-uk.com/1988ACT.php
"1. Section 141 of the Criminal Justice Act 1988 (offensive weapons) shall apply to the following descriptions of weapons, other than weapons of those descriptions which are antiques for the purposes of this Schedule:
...
(i) the weapon sometimes known as a "balisong" or "butterfly knife" , being a blade enclosed by its handle, which is designed to split down the middle, without the operation of a spring or other mechanical means, to reveal the blade;"
This means it is defined by statue as an offensive weapon in just the same way as flick knives etc.