Well Ed I wouldn't worry too much or take much notice of what has been posted so far there appears to be some big egos combined with old farts who have forgotten how little they new after a couple of years.
After completing L2 NVQ you are deemed to be able to install but still under guidance from a plumber, of cause what will depend on your experience so far. Level 3 is designed to allow you to design, set-out, commission & fault find. In addition you will get to understand un-vent systems, water reg's requirements & many other more complex systems as well as electrical knowledge that will far so pass most of the plumbers you are working with. Do your best, work hard, it is what you are like after the 4 years that is the most important thing.
You only get this opportunity once so make the most of it !!!