Painting Internal Brickwork over previous paint

So. I have an outside toilet in my place, which is where the boiler is (nicely out of the way). By outside toilet, I mean the door of the room opens into the garden, while two of the other walls are internal to the house.

In this room there is exposed brick work, which I plan to leave exposed (not paying to hide it away!). This has been painted in the past (no idea what with) and the paint has cracked and flaked off a bit. I have repainted this with some basic emulsion (2:3 water : paint mix) with two coats.

I have left it for about a month, and the paint work has cracked again. Possible things to note are the cold (it has been a bit nippy at times and there's no heating other than the boiler's frost checker) and the weather has been damp (especially when putting the paint on, but that was outside, I was working inside so thought that would be OK).

Now, the options as I see it to stop it cracking and flaking off is some combination of:
- Remove all old paint (wire brush, pressure hose or similar)
- Seal with PVA type stuff
- Add more coats of emulsion mix
- Use pure emulsion
- Use proper (expensive!) brick paint

Yes, I plan to paint over the emulsion mix with pure emulsion (bathroom paint left over from my parents' painting) once solved the flaking issue.