There is no requirement to have your RYA coastal skipper exam invigilated to become an instructor. This is because you will have covered the requirements when you obtain both the Yachtmaster certificate of compentence, and through completion of the instructor training program.
The instructor training program requires a solid fundamental understanding of the content, and it is highly inlikely that you could cheat your wa through this as your examiners are your peers in the industry.
The RYA scheme does support instructors from other trainging programs, however you will still have to complete the RYA scheme first to qualify in order to ensure mastery over the content of the RYA syllabus.
requirements and ensures a minimum uniform standard.