Does anyone know how many "one unders" are deliberate acts by third
parties?

I don't think it's that many, but not unheard of either. Off the top
of my head there was the Vic line one at Highbury last summer (the guy
was pushed under the train), there was the Picc line one at Earl's
Court (I think) where a bunch of students were messing around and one
of them plus a bystander went under a train (unfortunately the
bystander died). There was one on the Central at Mile End a couple of
years back (another one who was pushed). I'm sure there are others
which slipped my mind.....I think the majority of one unders are
either suicide attempts or accidents, but acts by 3rd parties do
happen, and I think the police have to consider all possibilities.