read the article again. The article is not actually comparing the heights of the Brit royals versus slick Tony and his wife. If that was all, then there is no possible libel and defamation suit. rather, the tone of the article is to poke fun at the President of singapore. They even admit it, when they say that using the word President instead of Minister is not the same funniness. In fact, the article demeans and humiliates him.
It can also be construed to mean that by calling slick Tony "Prime Miniature", they are insinuating and implying that he is a mini me version of the Prime Minister. And it therefore implies that he has the same powers and authority. This is not true, as his post is supposed to be distinct from that of the Prime Minister. The fact is that if Roy says this in singapore, there will be a lawsuit against him. So, if Tony got any balls, he should sue the Brit newspaper.