I'm not entirely sure about LARP swords, but I do know that recent legislation has made it less advisable to carry round, say, rubber pellet guns that look anything like real guns. (This made the university Assassins game a bit tougher. People just have rubber band guns and water pistols now, though.)

I don't think they're illegal, but I do know that a few members of my LARP group have had the police ask them what they're doing with these dodgy looking weapons (http://www.srcf.ucam.org/tt/wiki.pl?WhenPoliceAttack). So I'm just trying to be sensible. :)