Actually my point of view is "Once a Jolly Hangman" and "Once a Jolly Swagman" may have the same fundamental irony. It is probably written in a specific manner of the word Irony: writing which is intended to communicate a meaning contrary to its literal sense; contrast between what is expected or desired and reality
Without reading both books one cannot call it plagerism, though i can identify with the British irony, which can be pretty damn funny at times and not so funny at other times.
Many people around the world wouldn't really understand, as it is native to the roots and environment. In my old City of Lancaster the hangings took place at hangmans corner a part of Lancaster Castle.
Hanging in UK stopped in 1965, the last hanging was in 1964 I think. He should have toed the line, or walk the tightrope, without falling off, but he didn't, he was the pillock that got caught for a reason.
http://www.capitalpunishmentuk.org/hanging1.html
British sense of humour can be very uncouth, and i can imagine that many UK people would buy this book, just for the title, it's a little like nosey neighbours, when someone gets run over outside the house, they are not really interested in helping and could be very sarcastic for the crack....typical working class language, they could also burst out laughing because the person wasn't quick enough to get out of the way, their emotions are not touched so it doesn't really effect them.
Of course the choice of Title was chosen for the controversial response, many people would see the humour of it and others would be shocked by it.
I have no idea how one can find the balance of sarcasm without taking it to a vote, and to be fair, it would have to be a vote from a selected jury of Brits.
Singapore he's lost his case, the cane will not be used, he gets a great deal of publicity back in the UK with a population of 61 million,...he will not apologise as this will be just forgotten, he knows he will not be caned because of his age, the only place to hit him is financially, though how, as the money will be abroad. If he's jailed and his health suffers and he dies, he becomes quite famous.
My only suggestion would be humiliation by making him walk around with an advertising board, saying what a pillock he's been.

Then the historians also have a laugh at him. The British would remember him as "The Stupid Pillock" that wrote a book about Singapore he should have known better. I see a lot of $$$$$ in this book because of this publicity stunt which i believe it is, to a certain extent