The official guidance given to HMRC staff reads as follows:
"The Criminal Justice Act 1988 (Offensive Weapons) Orders 1988,
2002 and 2004 prohibit the possession and importation of:
. . . .
the weapon sometimes known as a “balisong” or
“butterfly knife”, being a blade enclosed by a handle
which is designed to split down the middle, without the
operation of a spring or other mechanical means, to
reveal the blade".
For future reference, with regard to any imports which you might be considering, I suggest downloading that document:
http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageVAT_ShowContent&id=HMCE_PROD1_025807&propertyType=document