I'd suggest hanging about school 'till you can legally leave (end of year 11) have a go at the exams (might suprise yourself)
It's better to come out of school with some qualifications than none, you'd kick yourself if 5 years down the line you're trying for a job and end up back at a collage doing gcse's to get a better one.
Also, are you going to get any sort of qualification (NVQ etc) from working for 4/5 years with your uncle? IF you are, great as you'll then be able to show you have these skills, if not, then you won't get a job anywhere after this 4/5 years, you'll have the experience, but nothing on paper, which unfortunately, the big businesses (e.g. British gas) and others will be looking for when hiring plumbers etc