David Cameron’s best pitch to voters ahead of May’s election is his stewardship of the British economy, which is now firmly among the developed world’s best performers. So it’s strange to see the Tory leader and his Chancellor, George Osborne, turn away from a key ingredient of their success.

That’s the takeaway from Wednesday’s Autumn Statement on the budget, in which Mr. Osborne unveiled plans for a “Google Tax.” The measure, technically called the diverted-profits tax, would hit multinational companies with a 25% tax rate...