All airports need to have surplus capacity if the operation is to be robust, because poor w/x or strong winds lead to flow restraints as there is need for greater separation. LGW tends not to have them as they alternate T/O & LDG and thus the separation is built in. A second R/W there would not double its capacity, just as a third R/W at LHR if it were ever built would not increase the capacity by 30%.
A country needs multiple communication links, the expansion of LGW would increase the UKs robustness and I believe the Freight capacity since it is closer to Europe and freight could flow back through the Channel tunnel as well into the heart of England