It depends. There is a lot of confusion by the average home-owner over what plumbers do and what they are. For example, traditionally a plumber deals with wet-plumbing (i.e. anything that involves water), but not gas. Gas is reserved for heating engineers. The average homeowner rarely uses the term heating engineer and refers to a plumber for both water and gas (e.g. boilers). In this case, a plumber definitely needs to be regulated, and is. Those plumbers (heating engineers) who use gas must be Gas Safe Registered for a start.