If you just want something little extra to keep afloat you'd be better off working in a supermarket (if you can get a job there) you get contracted to a certain amount of hours, and if they can't give you that many they still have to pay you for them, you're guaranteed National Minimum Wage and most have good schemes where you work your way up to managerial positions and nice bonuses.
As a self employed plumber (specially starting out) you'll work every hour possible, be constantly tired, struggling even more so to keep afloat (setting up costs) and I doubt you'd be making enough after NI contributions, van insurance, public liability, advertising etc you'll be making yourself national minimum wage. Its the wrong thing to be looking at if you only want to make a little extra. Sorry if this seems blunt/harsh but its the reality of it.
