Tom Gardner wrote:

An obvious point: the water you add to a plant will end up
in the air, except for the small amount that remains in
the plant itself.

[..]

I don't see how a plant can act as a useful dehumidifier
in normal UK conditions.

Nor do I.

If they could, then where would the water go and what would be the
physics/chemistry mechanism?

You answered your own question above. School botany was a long time
ago, but I remember that plants both respire (which gives off water) and
photosynthesize (which consumes water). They do the latter more, so the
overall effect is to consume water. But sadly for most plants you keep
having to add more water to the system to keep them alive.

Anyway, some plants can also absorb water from the air, so the only
question is whether you could somehow harness this to make a measurable
difference to humidity - presumably with a plant that requires minimal
additional direct watering. I think not.

The best solutions are to add less water and/or to ventilate.

Quite.

Peter