When working with a specific woodcare system use the right preserver for the system. The Dulux Weathershield system requires you to use a Dulux Weathershield Preservative Primer otherwise the whole coating life is not guaranteed, ditto with Sadolin.
Barrettine make great value but excellent quality wood finishes. They are based in the southwest as we are and we have traded with them for 20 years. We highly recommend them.