Part P guidance here
https://electrical.theiet.org/buildi...art-p/faqs.cfm
Basically anything other than 'minor work' needs to be reported to planning control unless you use an electrician registered to do part P work.
'Minor work' is defined in the link. Regardless of the reporting requirement, the work must be done in a competently.
Adding an additional light to an existing cct seems to me to be to be a minor work to quote from the reference above:
- however this is my opinion based on the above quote, and I am neither a lawyer nor a person registered to do Part P work.Examples include adding a lighting or power point to an existing circuit, adding a spur to an existing circuit or replacing a light fitting.
If you are in any doubt, seek qualified advice.