Stansted alone has at least 10m, possibly 15m, of unsold capacity available for the growth of point to point leisure/vfr travel - something it already serves very well.
Completely agree with this point, IM. This spare capacity is available immediately. However, it is reasonable to consider the probability that air travel demand will continue to grow between now and the earliest reasonable in-service date for a new runway in the SE. Both LHR and LGW can only grow from todays levels at the margins in terms of runway capacity and by encouraging use of larger aircraft types. So STN and to a lesser extent Luton are the only reasonable release valves for SE growth between now and a new runway opening. Barring a massive economic recession, that 15m at STN will be swallowed up well before then. And new demand will extend beyond the leisure sector alone. That isn't much headroom for accommodating 10-15 years of interim SE market growth.