I always thought nucular (for nuclear: hard to pronounce!) was an American usage problem. Though I must admit I had never heard of it until 2009, when I was at a conference on prescriptivism in Toronto, organised by Carol Percy, and where I heard Don Chapman give a paper on the topic. The word is mentioned (but not discussed in any detail) in Steven Pinker’s The Sense of Style as well.
(There are many more and very funny images online if you search for the word.)
But comparing Pinker’s work to some British usage guides, I found that nucular occurs in Caroline Taggart’s Her Ladyship’s Guide to the Queen’s English (2010) as well. And as a British English usage problem, it is much older than that: Robert Burchfield lists it in The Spoken Word (1981) (“not to be pronounced like circular“), and it may also be found in Greenbaum and Whitcut’s Longman Guide to English Usage (“Pronounce it like new clear, not as three syllables with the last two like circular“). They might had added that the stress would fall on new instead of clear.
And meanwhile I’ve also come across it – with a twist – in a British English collection of stories by Hilary Mantel, called The Assassination of Margaret Thatcher (2014). There, we find the following piece of dialogue:
… The walls are built to withstand a nucular bomb.”
“Nuclear,” she [an immigrant applying for a job] said.
She saw the look that flitted across his [the butler’s] face: don’t you correct my English, yellow bitch [p. 243].
So: not American after all, but so well entrenched in the British usage guide tradition that Mantel expects her readers to get the point.
This explains why Kiefer Sutherland pronounces it as Nucular in nearly all of his acting roles where he talks of nuclear attacks, especially the series, 24 Hours. He was born in London.