In short, the British pronounce “Z” as /zɛd/ (zed) whereas Americans pronounce it as /ziː/ (zee). Note that the same pronunciation is naturally used also in the plural: the plural of “Z”, denoted “Zs”, “Z’s” or “z’s”, is pronounced as /zɛdz/ (zedz) in the UK and /ziːz/ (zeez) in the US.
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