First thing is to consider regulations and legal requirements:
It is notifiable Part P work if you are DIYing so look on project pages for part p or needs to be installed by an electricain that is registered on a domestic installers scheme.
The requirements for cable underground is at least 600mm in an area that may be disturbed ie garden tools etc or at least 400mm if concrete path/driveway.
The cable should be layed in sand so no stone/bricks can damage and be covered with coping stone and yellow/black indication tape over that.
This circuit should be RCD protected either at spur as far as light just link it up to circuit.
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