Early in my start at DIY I needed some gas pipes changed. A British Gas engineer did the job, turn the gas back on, left and I discovered a cut off gas pipe unstopped pouring out gas. I decided that I needed to learn about gas work and did my own for years after. Now the government and trade bodies insist on training, certificates and registration for work on gas systems. Many other trades need certificates for commercial work, it keeps the training bodies well funded, but often you can do the work for yourself even if it needs mandatory checking after.