Do you have the official figures which state fewer than one commuter in 20 from Brighton to London gets on at Brighton?
For a start, commuter trains from Southampton to London don't go via BML, and only one per day (05.47) from Portsmouth does.
The principle issue for BML is the lines south...
The main problems of more trains per hour is a not-dissimilar one to certain sections on BML; there are only two lines - one up, one down. A fast train can only ever be as fast as the slow or semi-fast trains ahead of it.
Plus not all stations along the West Coastway can handle 12 carriages.
Not true. The Lewes end has been built upon, and they would need to either re-route it or knockdown the civic buildings and private housing built on its route.