IMO I wouldnt exactly buy cheap emulsion, you have just spent out a fair whack on getting your HSL skimmed, and the last thing you want to do is to put on some cheapo stuff, then put on the good stuff.
Spend a few pounds more and get some good quality paint.
Buy some white supermatt, or any white emulsion made by a good company, non vinyl is best as a base coat/sealer/miscoat.
Thin down by about 30-35%, then put that on your walls.
(Please dont use PVA)
This will seal your walls, and show up any imperfections left by the plastererer, (and there will be some), maybe not a lot, but there will be some lol
if you want to use a vinyl matt on top thats okay, or your choice of colour in a vinyl matt, but if going for Dulux, then I would still slightly thin down your colour by about 10-15% for your first coat of colour.
Then your top/last coat of colour, I would still add a tad of water, you will get a nicer smoother finish.