Lots of interesting points being raised.
I wonder though if this potential order is because they have realised there isn't enough units to provide a service once the Thameslink trains get diverted from London Bridge in December 2014 as the increase in journey times will require additional stock to maintain the timetable - especially with providing a Brighton Service from London Bridge.
It is noted the announcement clearly states that the extra units requires "agreeing consequential amendments to Southern’s franchise agreement" so I would suggest that means the units are for Southern itself.
Remember that the consultation stated joint services with Thameslink and Southern from Dec 2014. With the likely timetable (partly discussed on here) means they are likely to share stock and services. Note the London Bridge - Brighton is off peak, more units will be spare then.
Also important here is after Dec 2014, London Bridge low level will be completed allowing Southern to run as normal but some services will have to leave until displaced onto the Thameslink Core. With a new timetable being written things will change.
However it's the following making me thing the eight 5 cars are for cascading and helping release 319s for the North West.
Southern Managing Director Chris Burchell said "I am pleased that Southern is able to assist the Department in introducing additional capacity. The potential new order for 116 carriages will be exciting news for train manufacturers and for passengers".**
"Agreeing consequential amendments to Southern's franchise agreement" could mean They expect the franchise to last longer due to the problems in new franchises and expect Southern to still be around to introduce the 116+100 units. Remember under the pre WCML timetable Southern's franchise ended in July 2015 (barely 7 months after the additional eight 377s) and then are supposed to merge into Thameslink. So if Southern are doing this, they would likely need an agreement to hand over all work o the next franchisee which would mean amended franchise agreement.