Whilst idont think there are any serious plans about uckfield to Lewes there are plans for that region. Thameslink currently have pathing rights to places including Eastbourne, grinnie and Horsham but for various reasons don't run them...yet. Platforms are currentlymbeing lengthened for the 12 car Thames ink fleet on the grinnie line-sanderstead, upper warlingham and oxted are all complete and commissioned but the power needs updating before more than 8 can run and the signals need an upgrade to allow extra trains. The idea is that when they run 24 or however many trains an hour through city TL they can't all run on the BML so will need other places to go due to capacity.
As questioned earlierin the thread the reason southern run long distance services into Vic off peak is because vic is the main terminus of the Brighton line. London bridge is purely a commuter station for suburban services. To ease congestion at interchanges such as east Croydon they run limited coastal services in the peak.
I am told that over 100% of the track is used in the peak on that region which is why there are so often delays and it seems to be getting worse with extra services such as London overground etc.