Translation:It was spring last year that they went to London.
I wrote something similar myself that wasn't accepted. I suspect that for the best translation of the 是的 structure, they may be looking for extraposition in English, (that is, movement of the subject to a point father back in the sentence, while making 'it' the new 'dummy' subject): It was ... that (/when) ... // It was he who, ... etc.
That is what I considered the most common English phrasing, but it carries an ambiguity--'last spring' could mean the spring of this year when it's said in December. 'Spring of last year' would probably the clearest way to say it.
this is a little bit of a weird translation. "They went to London last spring" would be a little easier on the ears?
Certainly that is the basic, neutral sentence (or as a couple of people suggested, using 'spring (of) last year' to avoid ambiguity). However the point of the 是...的 structure is to highlight, or emphasize the part of the sentence immediately following 是. So, such a sentence is by definition, not a neutral sentence, in which nothing in particular is emphasized. The sentence is quite likely the answer to a request for that very information: When did they go to London?, or expressed in a cleft sentence (mentioned elsewhere here), When was it that they went to London? Lawyers and other people (a skeptical parent or girlfriend?) wishing to establish or verify specific pieces of information are particularly fond of the cleft sentence question. :-)
To answer this in spoken English, we might simply say 'last spring' louder. Putting 'last spring' first, as one person suggested, makes the sentence sound more to me like the answer to 'What did they do last spring.' These are some reasons why the 是...的 structure is often translated into English using the structure: ',It was X, that ... '. This places the emphasis clearly on X.
Really! Is the Mandarin not difficult enough, that they want us to figure that out for a translation?? Why?
Sometimes I write an answer and know its wrong because it seems to correct
Here is yet another example where thr "translator" has used English that is not quite correct. We would not normally say "It was spring last year...". More normally we might say "It was (the) spring OF last year." It is still not to late to have the translations checked by an educated speaker of English, and thereby reduce the continuing embarrassment.
I find both the short and longer forms acceptable, though possibly a bit different in formality or register. When I first arrived in Japan many years ago to teach English, I remember pointing out some 'errors,' that I soon discovered were normal expressions in the UK, but until then, unknown to me, a native of the northeastern US.
This kind of passive sentence, sounds very awkward and wordy in spoken English. More natural to say, They went to London last year in spring.