plaster over asbestos?

As long as you don't disturb the artex by scrapping it you can plaster over it. If it's a heavy pattern then I suggest lowering the ceiling and reboarding.

I would Recommend Building a Floating ceiling With timber and new plasterboard (pink plasterboard) lowering the ceiling around 3.5 inch approx just so you don't disturb the original ceiling if it turns out to be asbestos ,

We have done works in similar environments and our best course of action was to create a new floating ceiling so as to avoid any disturbance to the potentially asbestos artex .

if its not aspestos you can scrap off the prickles unibond twice one in the morning and one in the afternoon is fine then apply the next day 2 coats of multi finsh however weather its aspestos or not I would plasterboard over the artex as its a safe job and a lush finish everytime ive been plastering 35 years and I still get an artes ceiling giving me a problem ie when you think you have prepped it thouroughly scrapping off the loose you then can apply the inibond and when plastering and then finishing its reacts and you then have to scrap off your work and the next layer that didn't come off at ist plasterboard over everytime

As everyone els has said best corse of action is to lower the celling if it is asbestos you don't really want to plaster over it or over board it and run the risk of disturbing the original celling. Good luck