Complex sentence: Blair is concerned the 16 million voters who wanted to remain merit attention, if the deal with the EU that is worked out means leaving is not worth while or leaving has serious costs there should be a way of avoiding Brexit.
Split into simpler sentences: Blair is concerned the 16 million voters who wanted to remain merit attention. If the deal with the EU that is worked out means leaving is not worth while or leaving has serious costs there should be a way of avoiding Brexit.