Hey Ken, barrier coats are a pain in the proverbial.
I used one type on our 51 chevy and it too reacted with the base colour, so stripped it, etch primed it, and then used a different brand of sealer barrier coat. That one worked fine with the base colour, and we managed to get the truck body finished.
If youre using cellulose spray from halfords, then dont bother with the barrier coat. Just buy a can of grey etch primer, give it a light coat, and then leave it to cure for 24 hrs. Even though it is touch dry in an hour, it needs time to cure out properly.
Then you can go ahead and paint straight over that, might need a few extra coats of white to get a decent gleam over the grey, but it should work fine.
A quick tip to get better coverage with any halfords paint is to obviously have a warm area to work in, but leave the can in some hot water for five minutes, then shake it for a good few, and you get a way better finish with it.