Back in the mid-2000s, the London Stock Exchange was a reluctant belle at a merger ball, pursued first by Germany's Deutsche Börse and then Nasdaq OMX. The LSE evaded would-be suitors, only to watch deals create stronger rivals.

Fast forward to today, and the LSE is again being left behind. No sooner had it announced a merger with Canada's TMX Group than the deal was eclipsed by a planned merger between NYSE Euronext and Deutsche Börse.