Why the EU needs Britain as much as Britain needs the EU

On 7 May, Britain could take a decisive step towards leaving the European Union if voters re-elect a Conservative government committed to holding a referendum on the UK’s continued membership of the EU. With polls showing a 50-50 split between those wanting to stay and quit, the prospect of Brexit is more real than at any time since London joined the Brussels-based club over 40 years ago.

Some relish the prospect of saying auf wiedersehen to the UK. In a recent French poll over half sai...