"It was spring last year that they went to London."
There is nothing wrong with the English sentence and you neglected to include the Japanese translation. 'the spring of last year' would be better, but no big deal. Incidentally, why are you invoking Hanzi? Is there a Japanese for Chinese speakers course combined with this discussion board??
(かれらが）ロンドンに行ったのは去年の春でした。Would be appropriate. What do you have?