First advice is, whatever you try, do it on a test piece first! And leave it for a day or two to ensure there is no reaction with the enamel.
Having said that, my inclination would be to use Ronseal Hardglaze Polyurethane Interior Gloss (not the Acrylic stuff). Don’t use the exterior variety as it has UV agents in it which give it a yellow tint.
You should be able to spray it without any problems if you thin it down as bit as you suggest. It’s pretty thin anyway.
Humbrol do their own varnishes which you could use but I’ve heard that they are not as tough as Ronseal for a working model,
Steer clear of spray cans as the solvent in these frequently reacts with enamel.
Colin