In general it's fine to overcoat paints with a clear coat. After all this how cars are painted, with a clear coat as the final layer to protect the paint and ensure the best long-term performance.
The paint and clear coat don't need to be of the same type, or "share the same chemistry" as it's often put. You can overcoat with anything that won't cause the paint to craze or wrinkle, but I presume you'll want to keep the colour unchanged as much as possible and have application be similar to the paint's so you're limited to only waterbased finishes. Waterbased clear finishes are mostly absolutely colourless1 and if you match sheen fairly closely will minimally change the colour of what they're applied to2.
As I say in my Comment above, your real challenge will be in finding a clear waterbased finish that doesn't share the same characteristic as the Dulux paint you've used! You'll need to rely on good feedback from consumers and/or a finish comparison test done by a magazine or website to find one that has zero problems with remaining tacky. And while you're at it you might as well pick something that tops the charts in as many other respects as well, aiming for the best durability in the long term. With luck you'll far exceed the lifespan of the Dulux paint by itself, possibly adding years.
1 AKA "water-white". But do note because the finish is a suspension in water they are milky in the container.
2 Changes in gloss change apparent colour. If you gloss coat a matt paint in a pale colour expect that the colour will become slightly 'deeper', i.e. darker and more saturated.