Before you go mad with it, there's a simple check that will tell you if it's "polishable".
Drag a fingernail across the scratch - if you can feel it catch, you're (probably) through the lacquer to the colour.
Also, if the Autoglym SRP doesn't shift it, get hold of some G3 cutting compound and (carefully) do the same with that. As you use it, the compound gets dryer and seems to cut more. This should get rid of it in no time, just go steady with it.