Tom,
The Humbrol varnishes are capable of giving a match to the original Hornby finish (which I presume is the intention) but it is much more to do with application than the brand of varnish. A very fine sprayed on coat is what is required and you need to experiment first on scrap samples to find which particular varnish with your technique does the job satisfactorily. Trying to do it by brush is much less controllable. My starting point would be with an eggshell varnish.