Brian Micklethwait nails it, in my judgment. What he says has much broader application than just to the British Isles.
Here in the US I can remember Yellow Dog Democrats and true blue Republicans. Political brand loyalty was much, much stronger in my childhood. The irony is that while the parties forty years ago were essentially similar, the party loyalists were not. Today, the parties are ideologically a bit more differentiated and the base for each is more similar to the other, with a much larger pool of voters who are willing to switch parties given the proper circumstances.