Well theres dark and theres dark, when you have ertain plants on the night cycle of the day, i would put them in total dark, pitch black, it stops them stretching and produces a shorter bushier stronger plant. So the airing cupboard will be good for that.
The problem is air flow, warms good but lack of air flow isnt.
If on the window sill in the kitchen they get nice fresh air and a healthy breeze, then the plant can breath properly and the branches will be moving constantly thus making them stronger, a bit like lifting weights for us.
Although you will get light polution lol.
Do a test, place one plant in the airing cupboard and one in the kitchen each night at the same time and see which benefits the most.
The only time a plant really needs extra heat is the germination stage and maybe during its daytime. At night it gets colder, the sun goes down.