Hi DJH,
I am sorry Britain doesn't have a vapour barrier paint standard or certification or market plainly marked vapour barrier paints, which would have made it easy for you...
Obviously, alkyd oil based paints are great barriers but lousy to clean up and have VOCs which will linger in such a small space long after painted. An alkyd garage concrete floor paint would be a good choice, another awful to clean but durable non-slip option are epoxy floor finishes with texture made just for garage and shop floor.
On the easy cleanup side of things you have your emulsion paints, or what we in North America call latex. They come in two bases, the first is vinyl which is not very durable on floors and not moisture vapour resistant... so forget cheap basic emulsion paints...
The next are emulsions based on 100% acrylic resins. They are more durable and water resistant. Two coats of a 100% acrylic emulsion paint should work just fine for you... throw in some sanded grout mix into the paint and stir well. Apply with a roller and you should have decent non-slip characteristics as well.