I'd do the base coats in craft store acrylics. Thin them to the same consistency as your normal GW paint and they will work fine. Apply with a big brush. Use two coats if it's a colour that doesn't cover well. Use your good GW paint for the detailing, or drybrushing even. It will not be any harder than basecoating with GW paint, it won't stump your artistic ability, and it will almost certainly be a good deal quicker. :P (And definitely cheaper.)