There are other reasons to have qualifications and certificates. Other than the idea that they are there to exclude amateurs. I think the new certifications for electrical and plumbing seem to have coincided with improvements in the materials and products supplied by the industry. ie electrical work now involves different colour wiring. plumbing has seen the introduction of condensing boilers and a lot more eco related stuff. So this has prompted the requirement for certification to compliment a renewed enthusiasm for the job. All of which benefits the manufacturers and suppliers. There is not such a comparable development for carpentry work. But maybe if there was, there would be a renewed requirement for certifications.