It's actually NOT perfectly legal to have a locking blade on a multitool. To the best of my knowledge, there are only two multitools that are legal for day to day carry in the UK. One is a victorinox with a non locking blade and the other is the Leatherman knifeless fuse.
Carrying a blade that locks open, regardless of length is illegal unless you can prove that you had a good reason to have it with you. It is an absolute or strict liability offence, and the police do not need to prove intent or guilt in any way - you are automatically guilty unless you can show a good reason for having the item with you at the time you are arrested.
Having a multitool on the way to work where it will be used is acceptable and defensible. Having one with a locking blade at the pub on your way home from work is NOT acceptable or defensible under UK law. Ministers have admitted that they never intended for this to be the case when the knife laws were drafted, but at least two people have been convicted of knife crime for having a multitool on their person and not being able to show a reasonable excuse for having it at that time - again, habitual carry is not an accepted excuse.