Stroma. Part P is building regs exam C&G 2392 or something close. It's a one day course. You have to test and verify any installation prior to commissioning. You have to issue a certificate for most work. If you do not join a scheme you will have to notify B.C. local authority and the client will have to pay hundreds. If you are with a CPS, they will arrange notification and the client will receive that important document as passed by B.C. saving hundreds circa £300.
So you will need CPS membership. Each one will require Qualies, differing with each. They will need insurance PL and for EICR PI. They will want to see your complaints procedure, H&S statement Risk Assessment and method statement. Also some examples of work.