At that point in history, British Telecom was a state monopoly, and even by 1982 BT only allowed (via approval) the four British manufacturers (STC, GEC, Plessey, and Thorn-Ericsson) to supply its twenty five types of phone through them, and not independently.
Since that date, IMEIs have been allocated by BABT (or one of several other regional administrators acting on behalf of the GSM Association) to legitimate GSM terminal manufacturers without the need to provide evidence of approval.
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