Several websites, including gov.uk and BBC Wales, say that James Callaghan was known as Big Jim, but (having been around in the 1970s), if you'd asked me what his nickname was, I'd have answered "Sunny Jim". This is given alongside Big Jim by the Government site.
The original Sunny Jim, as we discovered a few weeks ago (well, I did anyway) was the logo or mascot of a breakfast cereal, produced in the USA and first imported to the UK in 1902.
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