You're in luck - the Airports Commission did consider a proposal in respect of Manston:
"Policy initiatives and surface transport improvements to develop Manston as a ‘reliever’ airport for London and the South East, freeing up capacity at more congested airports, and reducing the need for new runway capacity to be built."
Their verdict:
"This proposal presents some potential as a reliever airport, but does not address the larger question of London & South East capacity. The concept of reliever airports is considered in short and medium term work. Please see Appendix 1 for further information."