You want to have one socket and one light so that there is a circuit. There are five certificates:-
Installation certificate - in the main with domestic, this will also need another certificate as well.
Minor works certificate - often does not require another certificate as well.
Electrical installation condition report - often raised where there is neither of the above, and the LABC wants proof that the installation complies, and often the LABC will stipulate who they want to do the report.
Compliance certificate - Issued by scheme providers to confirm the electrician who raised the installation certificate was a scheme member.
Completion certificate - Issued by LABC shows they were satisfied that the work was done to required standard.
Some work only needs a minor works certificate and does not need registering with the LABC, so you want to ensure any work you do will not need registering as rather expensive, so as long as the circuits exist from the garage consumer unit you can extend them without registering the work, so you need to have something supplied from the two circuits, so you are not making a new circuit.
I don't know English charges, in Wales it starts at £100 plus vat, so you don't want to add circuits after the installation certificate is issued.
I have looked at the certificates issued for this house, and to be frank the details were very scant, and how anyone would know what was done from the certificates I do not know. I have done work under the LABC and submitted the installation certificate, they wanted to view the test equipment before they would allow it, and my qualifications, so although in theory you can DIY in practice you can't.