Can you blame them? Just think of the cost which First have to do to bring them up to FCC standards ie high back seats, refurbished interiors etc...
I think one of the reasons for the 313s not coming over is also because some of the current services which run as 6 cars can easily run as 3 cars so seeing as the capacity is there, I think FCC took a look at the stock and realise they didn't actually need the extra 313s.
But the other thing as well is there's no point in having the extra stock if you don't have the infrastructure to deal with them ie the signalling on the NCL to be upgraded and the upgrading of the Up Goods Line to passenger standards between Alexandra Palace and Finsbury Park will have to happen first before any increase in services is possible, don't forgot the new platforms as well at Hornsey and Harringay which when completed will be able to keep services from Welwyn and Hertford separate as there are some services which are limited stops in the peaks.
Not only this but it will help increase the capacity in the already congested section between Alexandra Palace and Kings Cross/Moorgate.
Still, I can't wait to see the backlash from the Southern passengers when they see their 377s replaced with 313s,