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They are three different things really.
First I suggest you apply for Electrician Eng Tech via the IET (I see you are already a member)
ECS Gold Cards are issued via the ECS (Electrotechnical Certification Scheme) google to find that.
Part P is for people engaged in electrical installation (domestic) who wish to self certify work without applying to building control.
As I said, three different processes, all requiring similar qualifications and experience.
There is no REW as such in the UK. The nearest thing would be the ECS scheme probably.