England only needed a point to be assured of victory following recent wins over Italy and Korea Republic.
But the Lionesses blew Belgium away with a breathless display of attacking football to register their largest win of the competition.
England led at half-time courtesy of a Kelly tap-in and William’s header, before Kelly became the tournament’s top scorer by reacting quickest from a Nicky Evrard error to make it 3-0 shortly after the break.
An unfortunate Julie Biesman own-goal followed by a neat Lucy Bronze finish made it 5-0 to the hosts, with Elena Dhont scoring from Belgium’s only shot on target to ruin Mary Earps’ clean sheet.
Williamson rounded off a near-perfect evening for the Lionesses with an exquisite finish from outside the box deep into injury time.