Satin,
I am a UK police officer with 28 years' service.
Any knife or tool which has a locking blade is illegal for EDC in the UK, no matter how long or short the blade is.
The only knife legal for EDC here is a 3" or less, slip-joint folder.
The carriage of other types of knife is legal only if you have a specific, current need. This might include an angler carrying a filleting knife, a ghillie with a 4" skinner etc.
Knives Act 1997
Offensive Weapons Act 1996
Criminal Justice Act 1988 (See sections 139 to 142 inclusive):
http://www.legislation.hmso.gov.uk/acts/acts1988/Ukpga_19880033_en_1.htm
Case law establishing that lock knives are illegal as EDC:
http://www.hrcr.org/safrica/arrested_rights/Harris_DirPubPros.htm
The Restriction of Offensive Weapons Acts 1959:
The Restriction of Offensive Weapons Acts 1959 makes it an offence for any person to manufacture, sell, hire, offer for sale or hire, or to expose or to have in his possession for the purpose of sale or hire, or to lend or give to another person a flick knife or gravity knife.
maximus otter