White paints often have a quite course pigment that makes it both spray- and brush paint more difficultly than most other colors. To avoid this, I would try to find a light grey out of the tin that is as close to the shade you want, further mixing other colors to it to darken, lighten or change the hue a bit if necessary. I would go for a warm off-white / light grey indeed. This will look like like white on a model indeed, and still gives you room to highlight detail until pure white.
Try to use your eyes to judge the shade you want, rather than mixture ratio's. But remember that the off-white or light grey will always look a lot darker and less like white in your mixing tin than on the model, so don't be afraid to exaggerate just a bit while mixing.
It should be perfectly possible to brush-paint light colors like this. Just thin the paint well and apply it in several thin coats, letting it dry well in between. The thing with light colors is that it takes more coats to cover properly than dark colors. With Humbrol enamels, it would probably take me at least 4 coats for proper coverage of such a color with minimal brush strokes.
Therefore, airbrushing would simply be easier and faster, and my first choice too.