The FAA issue a Type Certificate for an aeroplane, and then any amendments to the aircraft (or subsystem approvals) are promulgated as an STC. The British equivalent to an STC is an AAN, Airworthiness Approval Note.
Go to any avionics supplier and they'll probably have copies of the STCs for most of the kit they sell. It might also be worth giving the CAA's avionics dept a call, they'll be able to give you more of the chapter and verse than I can - the CAA switchboard no. is 01293-567171.
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