Oh dear, what is the matter with you? LEGAL? BS7671 is not a statutory document but it is a compliance REQUIREMENT. Similarly BS-EN 50131-1 is not a statutory document but it too is a compliance REQUIREMENT*.
I guess you don't have a copy (of either standard?)
Here's the relevant paragraph.
Electrical connection to the mains supply shall be via an un-switched fused spur point in accordance with the electrical installation regulations of BS 7671.
Power supply faults (including mains power outages) shall be recorded in the IDS event log and indicated at the user displays when the system is unset. No indication of a power supply fault should be displayed when the IDS is set. Power supply faults must also be notified to the ARC, although notification of mains power outages may be delayed for a maximum of one hour.
BS7671 SPECIFICALLY allows FCUs on circuits, including lighting circuits, using an FCU meets the requirements of both standards.
Will you shut up now?
*Here's the REQUIREMENT bit
http://www.nsi.org.uk/wp-content/uploads/2012/11/NSI-Technical-Bulletin-No-0040-PD-6662-2017.pdf