I understand your concern. When you say 'connecting a flex to a FCU' are you considering installation of said FCU.
I'm sure you would come across situations where this would be required. With the installation of such (fixed wiring), there are electrical requirements including installation testing and provision of a completion certificate, in this case a Minor Works cert. The testing would also require a basic check on the existing installation, in case of any problems. RCD protection is another factor to be considered.
If this is what you require, scheme membership would be advised, thus giving scope for further work, if required.