Everyone here seems very excited to try and answer the grammar questions haha! You probably know your grammar rules better than I do, but I believe that this is the second conditional (immediate-future time period using an unlikely hypothetical). It has been a long time since I focused on conditionals, but what might be confusing is that the conditional clause and consequence are being stated in the reverse order. The conditional (if Corbyn becomes) follows after the consequence (we should all move). "If Corbyn becomes Prime Minister, we should all move to China" sounds correct to me.

In context, this is probably not second conditional, but first conditional. In English, the word "should" is very often used with a present or future tense meaning, useful for making a ssuggestio, as opposed to stating an obligation with "have to," "need to" or "must."

Jon has it right. Dont be misled by the past tense in the beginning of the sentence. What he said is in the present.