~There is nothing stopping anyone from doing basic plumbing as a self employed person. By law, qualiforcations are not needed. Saying this, one should be competant before they do any job.
Go and see your college tutor because I think that you can still do the nvq 2 for free within five years of starting the 6129.
For information on how to be a self employed person go to Business Link or any accountant. They will usually give you advice for free.
At the same time it would be helpful if you could go to your local trainning centre and do the one day Water Regulations course, Unvented and Energy Efficiency.