AST
I'm currently on the AST residential ATPL ground school and would recommend it. Good experienced lectures on hand and for the AGK side of things it's excellent with engineering training on site you have access to their hangar with various aircraft, systems and components giving young greater overall knowledge. AST use pro pilot's pad pilot notes which are pretty good.
I have also heard good things about pro pilots ground school in Coverntry so don't think you would go wrong with them either. Don't have any experience with the other schools.