I'm guessing here although I work out of LHR, but I would think that monopods, even if they are not on the banned list, could arguably constitute a 'weapon'. The fact that people have got away with transporting them in the cabin, is, I think, simply luck. Even in a carrying case, they still represent a potential weapon.
If they won't fit in an overnight bag or (preferably) a suitcase, I would err on the side of caution and leave them at home.
I certainly wouldn't risk having my expensive Manfrotto tripod or monopod confiscated.