you need to let the plaster dry out on the surface at least. buy a cheap matt white emulsion and add 50% water apply this as a basecoat, once dried apply any emulsion, if your good at painting? the dulux you mentioned will be ok to use but if your not you'll find it a nightmare along with dulux diamond or jonstones ultra, these paints are excellent and ideal for high traffic area and if done right last ages and look fantastic but if your not very good you will end up pulling your hair out they have to be worked one way and not retouched until fully FULLY dry. your cutting in is the main problem you cannot brush back over what you've just done you will bring most of already applied paint off and it will go stringy.
i'd go for a standard vinyl matt and apply an extra coat probly be much easier and faster.
hope of some help nige.