In decades gone by, U.K.-headquartered law firms were an unstoppable force expanding across every part of the globe. But that force hit an immovable object in the form of the U.S., where the market proved too crowded and costly for U.K. firms to breach.

Then came the age of U.S. firm expansion beyond their homeland as well as a flurry of transatlantic mergers. The top-tier U.K. firms looked on from the sidelines seemingly unable to come up with a clear strategy on how to tackle America.