"Sparking" or "Ladies work"
if you like. It's a good money spinner and mixes with Plumbing at many points. So its worth while doing a course in it.
Mind you it depends how far you want to go with it? Are you going to want to work on the National Grid, build computerised electronic systems, commercial, industrial or just stay domestic?
Funnily enough most people even Plumber's on the website don't seem to talk much about going into the commercial or industrial Plumbing markets?
Probably the out lay would be to much. But there is usually plenty of repair work in these areas. However I suppose you woudl need a bit of bottle to offer the likes of a car plant your services. But hey! Why not?
As to a "chippie" or a "rough house brikkie" well its good to be able to do a bit of both, but you hardly need to be fully skilled in either, just get to know a few modules.
But perhaps expect protests off the other trades if you go over to much into their areas they want to make a living as well.
Tiling is another area as is glazing.