The London stock exchange rejected a surprise takeover bid from its Hong Kong rival on Friday, citing “fundamental concerns” about the price and purpose of the offer. More intriguing may be what the bid says about the costs of Chinese political encroachment in Hong Kong.

Beijing’s meddling makes diversification appealing for the Hong Kong stock exchange, but it’s an avoidable risk for London in an era of Brexit and trade tension. In its rejection letter, the London board noted that Hong Kong stock-exchange shares made up...