The Humbrol will give the hull a nice even finish but it is designed purely as a decorative finish and not as a protective finish.
To give some sort of protection you need a coating that is designed to protect and you cannot do much better than Polyurethane varnish, prefferably yacht varnish quality.
If you can get it in a matt spray form so much the better but if not then put some through an airbrush or mini spray gun.
I would always, no matter what paint systems I am using, always do a test piece first. Even when you think you know how paints are going to react you can get caught out so play safe and squirt a bit of the Halfords tin onto a sealed surface then, when properly dried, put some of the Polyurethane over it and see what happens.
And if you are using Matt polyurethane you cannot mix it frequently enough. The one thing that spoils more matt finishes than anything else is not mixing the paint enough. When your arm is tired have a rest then do it again. Even better put a paddle of some sort in your battery drill and mix it with that, until the battery runs down or until next Tuesday which ever is the later!! | 
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