You need someone competent, with the tools and equipment to install, test and commission the system. I've worked on a lot of self installs, and they generally need a lot of work to put right. Always see the earth cut out in the detectors, never see the cabling installed appropriately, and always installed using a switched FCU rather than the key isolator.
If it is your own house where you live then you may be happy to do it yourself.
If you are an employer with a duty of care to employees and don't want to fall foul of the HSE if something goes wrong then absolutely get it installed and certified by a competent person.
