Why use barcoat at all if you dont need to?
whilst i agree that most of the time you dont need barcoat you can never actually guarantee that you dont untill you see a reaction and then you know you did
as it is the op has some halfords areosol paint on the car which is junk so if it was me and i didnt have time to remove it i would definitely use an isolator
also if the op is unsure of paintwork, doesnt want to remove all old paint and doesnt want to waste his time and all his paint i dont see a problem with isolating it in the first place its a very thin coat and it will guarantee no nastys happen