Use the applicable publication
The IET publishes BS 7671 and supporting guidance for UK electrical installations. Check its publication hub for the relevant edition, amendments and updates before using a technical reference in a real design.
Consult the relevant publication for numerical tables and their conditions of use.
Build an assessment from evidence
The installation, protective devices, manufacturer instructions and test conditions all affect how a requirement applies. An individual calculation is one part of the evidence, rather than a certificate for the installation.