Revolving-door UK leadership leaves no room for remedy

British markets seem wearily resigned to a seventh prime minister in the decade since the seismic Brexit vote of 2016. But unless this latest government really delivers, ingrained political volatility will only drag on an already hobbled economy and further limit the space to fix it.

There is a sense abroad that the United Kingdom is increasingly ungovernable, with its main parties riven by factions and unable to agree for long on leaders or policy directions, despite popular mandates for...