Britain already fielded the Gloster Meteor in 1944

What would have to change for Britain to have an operational jet fighter during the Second World War?

Nothing fundamental: the Gloster Meteor entered service in July 1944 and conducted Allied combat operations.

Britain already had an operational jet fighter during the Second World War. The Gloster Meteor first flew in March 1943 and entered service on 27 July 1944. It was the only Allied jet aircraft to conduct combat operations during the war. A useful counterfactual would ask whether the Meteor arrives earlier or performs a broader role.