The FCC franchise is scheduled to finish in 2015 so they will get very little benefit from the new trains which are coming for the Greater Thameslink Franchise.
The new franchise will also run trains on much of the routes currently operated by Southern and quite a lot of South-Eastern, which will include Sevenoaks, Ashford and Dartford.
Judging by the services covered very little out of London Bridge will remain with Southern. Four trains an hour down through Redhill, East Grinstead Branch, Guildford Trains via Sutton, Horsham Trains, Eastbourne, Littlehampton and of course Brighton.
Southern will be left with Uckfield (Diesel and can't go to Victoria) and a few locals in South London that TFL haven't added to the Overground.