I think NVQ level 2 has to be viewed as a step along the route rather than a qualification. The reason that you aren't going to make a lot of money with level 2 is that you aren't going to be able to do anything special. Contractors and employers want people who are qualified to do the whole job not just part of it and level 2 doesn't cover stuff like unvented systems, combi boilers, gas supply and so on; common stuff in todays new builds.
To quote one of my instructors on the subject of level 2 - "It ain't worth a wank on it's own"
That's why I'm going for Level 3, to be able to do the whole job. Once I have gained sufficient experience, I'll go and work for myself because you'll never make decent money working for somebody else.
My tuppence worth

Mo