The overheads: buildings maintenance, management, support staff, support services, means it costs more to run two centres than it does to run one.
Of course any ATC centre needs a backup in case of a catastrophic event but the Prestwick centre could not handle the Swanwick business and visa versa.
That may well change should NATS's management ever realise their dream of virtualisation but that is some years off yet and way off the original topic.