If I'm reading things correctly, Porterbrook are not involved in the C2C deal, which somewhat limits the potential options available or was the comment regarding only their 387s?
The first 700s are due into service in April (?), allowing a few 387/1s to move to GWR, otherwise I can see why the 387/1s would have been the obvious solution. Unless this is changed, given delays to the GW electrification, those first few 700s could allow some 319s or 377s to go to C2C, with later 700 deliveries displacing the 387s required for GW's Hayes & Harlington services.
Either way, the only other manner I can see any AC stock being made available in the next few months is some convoluted deal/moves involving the 442s - the only other stock becoming available in the next few months, resulting from the 387/2s coming into service - April again? Are there suitable diagrams which the 442s could take over from 377s, of any variety on Southern services, allowing a shuffle around of 377 diagrams with some 377/2s being made available for C2C? From what I can recall, not all the 377/2s are required for dual voltage diagrams.