agree with
@Dustydazzler on this one,
Notifiable if you are adding a new breaker or connecting to a spare one that is not being used.
that would be under the definition of a circuit and it would be "new" as it was not there before.
however, if there is already a circuit running from a breaker, adding some more cables to it and putting as many sockets, lights, dishwashers, heaters, grow lamps, extractors, welders, etc. as you like, would not constitute a new circuit and therefore not be notifiable.
however, to comply with the wiring regulations you would still have to do the following.
design the circuit (including modifications) to comply with wiring regs,
ensure all work is to a good standard that does not reduce the safety of the installation
TEST the circuit including the modifications and record the results on a minor works certificate
provide a copy of the certificate to the person ordering the work for there records.
P.S. there is no get out clause if the person ordering the work is the same person that completes the work, it is still a requirement to record the results and document it.