"It was spring last year that they went to London."
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.
It appears the 是。。。的 is to emphasize details in the past. I wasn't familiar with this useage and looked ip the following link which might be helpful. https://resources.allsetlearning.com/chinese/grammar/The_%22shi..._de%22_construction_for_emphasizing_details