Two decades ago, U.S. policymakers fretted that London would eclipse Wall Street as the world's primary listing destination. Now, U.K. mega caps are moving their primary listings to the U.S., or deciding not to list in London in the first place.

Improving access to capital is the key to reviving the beleaguered bourse, according to London Stock Exchange's deputy chief executive Charlie Walker. "We've got this sort of mechanically sound car now, and we just need some petrol, and the petrol is capital," Walker said in an interview. Read more: