Well since you asked OBG :-)
A knife with a blade more than 3 inches in length is illegal. A knife with a locking blade is illegal.
It is illegal to carry a weapon. If you admit to the police 'yes i suppose i would use my leatherman (or paper, or umbrella or anything) you just admitted to a crime. Intent to use it as a weapon makes it a weapon.
But we have had a few cases of people being arrested with leathermans and being found innocent in court.
No british policeman will arrest a tourist who doesn't know the law (especially an american who we love) and no court would convict. You'd have to be unlucky to have it confiscated. That is the worst than could happen.
The Sock