if you run your own plumbing company i dont see why not.(i may be proven wrong) to get the nvq you are known as a qualified plumber and as its an nvq you have relevant industrial experience
jury is still out on this!
i'm pretty sure that as well as the necessary quals someone requires 5 years on the job experience to train somebody up. lets face it the nvq tasks are pretty simple and some of those who have passed it could still be like the blind leading the blind if they were to take someone on
as fuzzy says though there could be an important distinction between taking someone on and being a workplace recorder.
if you are allowed to take someone on it will however be 5 years before you can put him/her on an nvq course as you won't have the experience til then to sign off tasks for him. the alternative for said trainee is to get all work assessed direct by c and g but this can be a real ballache, because you try and arrange a time for a visit when you have suitable work such as a bathroom and 9 times out of ten they are booked up which is why it is so beneficial for your trainee for you to be a workplace recorder. otherwise it will take him up to twice as long to qualify