"It was spring last year that they went to London."
是去年春天他们去伦敦 should be correct unless I've been speaking my mother tongue wrongly my whole life?
Can someone kindly explain the construction of this sentence? Why the 是 and what is the 的 at the end for?
I don't think so, because in your example the 是...的 puts emphasis on 他们 while the original sentence puts emphasis on the fact they went to London in Spring last year. A translation to your sentence would be "They were the ones who went to London in Spring last year".
Hope it helps.