Sorry, and I mean know disrespect to the OP.
But reading through this thread your simply not ready to approach the task at hand.
Remember,
Being competent has never been about what you know.
Its about knowing what you don't know.
1st stage is to have the whole installation thoroughly tested as per an EICR.
Don't need to complete the whole form if factored into your price but you need to visually check the installation and record readings for your benefit.
Once you have identified any dangers or non compliant problems you can tackle the job issuing an installation cert after completion.
Part P registration is as much, if not more, for your friends benefit.
You need an established qualified sparky at your side and run through the job with him.
That's my honest opinion, and advice. :icon12: