Use a Halfords rattle can over Humbrol and you are likely to simply bubble the paint, I have the test pieces to prove it. The new Halfords lacquers might work on very old enamel finishes but certainly not on anything recent. They do seem to be OK over many acrylics such as Tamiya and Vallejo though. I have been testing them myself for my current project.
It is always best to use paints from the same manufacturer all the way through wherever possible to ensure compatibility otherwise you take pot luck. Even rattle cans from different manufacturers can be a problem as they often use incompatible solvents. So if you paint your boat with Halfords cans and then apply a varnish from somebody else you can often get a bad reaction.
I have never had any problems with good quality Polyurethane interior varnish yellowing in over 40 years myself. The boats simply don't spend protracted periods outdoors in sunshine. If you use the exterior versions or yacht varnishes then you will definitely get yellowing immediately as the varnish contains yellow UV inhibitors and this really shows up on light coloured topcoats.
Above all, ALWAYS do a test piece, otherwise you will find, as people frequently do, that you are faced with stripping everything off and starting again.
Colin