I thank everyone for the answers. on how to test a system, in addition to paying the engineers, I also help them perform the tests and it is no problem to buy the equipment to perform the tests myself, to fill out the certificate I have no experience but I can do some research online. generally I do not do simple systems, use relays, remote controls, home automation systems and in shared house induction loops to avoid problems for people who have trouble hearing. what interests me and that if I make a certificate to a shared house, my client has no problem in delivering it to a building control or presenting it to a bank for sale.
for the rest, to return the favor of a certificate I have to go to work with them for £ 130 a day, otherwise alone I earn well, although much of my earnings take the engineers for the tests, I do all the work and they with a signature takes 1/3 of the gain. if I do not have a language problem, I would go to work for some big company or I could earn more on my work, but it is difficult to learn it in an all-Italian environment between clients and workers. as much as the Made in Italy workforce is well accepted for a nice, clean and safe job, not speaking English limits the clientele.