student visa, you can only work 20hrs a week in term time, nvq was in the past mostly a work placement (probably paid) but recently the rules were changed so you have to spend half the time in class room ( the gov knows the nvq was being abused as a type of work permit)
but work paid or unpaid is work, and your right it should be only 20hrs week, but seems to be a bit of confusion as whether your time on your placement is counted.