You appear to be getting crossed terminology here Kt.One the one hand talking about electrical certificates.
Other hand part P notifiable work! Two different beasties.
Long answer coming up. grab a coffee first :coffee
1/ Electrical work and certification.
All electrical work should be tested to verify its safe and your results written on a suitable certificate for the person ordering the work.
The essential minimum recommended detail is given in Appendix 6 of BS7671 on various model forms.
The various stages of verifying, inspecting, testing or writing up the certificates needs to be done by a competent person..
e.g.
Initial verification carried out by competent person Reg 610.5
Period inspection by competent person 621.4
Certificates written by competent person 631.4
So any person who has gained some suitable qualifications such as C&G's
can be considered "Competent" and is therefore able to install, test, certify their work in accordance with BS7671.
There is is NO obligation to be a member of any approved contractor scheme.
2/ Building regulations Part P
This introduced the concept that certain types of electrical work, (but not all electrical work), must be notified to a local council building control and proof that it has been carried out in accordance with building regs AND BS7671 should be obtained for the homeowner so they can then acquire a building regulations compliance certificate for the amended work.
ANY competent person can issue an electrical certificate to a LABC to verify that some Part P notifiable work has been undertaken correctly. The LABC would need to verify the integrity of the contractors claims that they are competent. (a C&G certificate may be sufficient evidence.)
So providing a competent person can issue the essential test information on a suitable form, similar to the model forms, and pay any relevant building control fees. then the LABC can then issue a compliance certificate.
3/ Test meter manufactures forms
Because you only need to be competent, some electrical test meter manufactures produce suitable certificates for purchse...
I guess the assumption being... a person purchasing a test meter is probably going to do some tests and will therefore need to record those test results somewhere?
4/ The Approved contractor scheme providers:-
To make the notification process "easier" various electrical contractor bodies.. (already in existence well before Part p came along),
introduced various membership schemes allowing their "vetted and assessed contractors" to self certify their own work and notify jobs via their scheme provider. This saves the LABC the hassle of checking the credentials of hundreds of electrical contractors.
For the same reason the meter manufactures produce certificates the various contractor bodies also produce their own certificates..
You can choose to use their certificates if you so wish..
but you don't have to!
however, whichever scheme provider you register with, some may say it can look more professional to your customer if you have an additional Logo on your certificates to help verify your competence with electrical work!
hope that sort of puts things into context?