I just prefer to control the pressure, paint flow and thinning myself. Some jobs need this. Don't get me wrong; the Halfords white primer is a good quality paint but aerosols are wasteful and expensive and using them as well as an airbrush is like having a dog and barking yourself.
You're right about mixing the paint. The matting agent really needs scraping off the bottom of the tin and the whole tin thoroughly mixing to the same consistency. I've even found that after a long period without use, the matting agent has once more stuck to the bottom of the tin. Maybe the main problems people are having are down to the fact that the old tins didn't need this level of extreme mixing. At the end of the day the paint is just nowhere near as good as the old stuff but decent results can still be acheived with the new paint. I'd say don't throw it in the bin.