You probably could make it that way, except I don't know off top of my head what other substances there would be (CO2 most likely), and above all, sodium ascorbate is used for IVs or as drops for sinus problems and the stuff you'd make at home would not be pure enough for that type of use. I'd stick with commercial grade.
if it's too expensive in the UK, have you checked prices in the US? I use calcium ascorbate so I wouldn't know.