The shortage of 700s is purely down to reliability. They were ordered to have an MTIN of around 40k. Reality is closer to 13k. I guess they have a while to sort this but I understand they may need to at least temporarily bring in other stock on the London - Cambridge so the 700s can cover elsewhere.
seen the 379 rumour myself. No idea why the 365s are not considered. Available now, drivers / fitters train. Move the shacks stops to Cambridge- Brighton to avoid SDO mods.
It would make sense if 700s are becoming due for maintenance and/or some kind of reliability modifications, and availability is insufficient to release units without causing stock shortage.
The easiest way of doing it would be to steal 6x387 from the peak Peterborough services, backfill with stored 365s (which to use an in phrase are oven ready!), and use the 6x387 to replace 3x700/0 off KX-Cambridge/Welwyn. The only potential snag I can see is if there's any traction knowledge issues with the 387s, both with who will be crewing them on the KX-Cambridge/Welwyn services, and in terms of taking them off the Peterborough route so reducing exposure to Peterborough crews. In the longer term it could then lead into keeping the extra 365s on the Peterborough route, and using the 387s to make up more 12-cars in conjunction with the 2tph to Kings Lynn (if that ever happens!).
I've always thought that the villages would be better served with 1tph off the Cambridge/Brighton, as this would give them an hourly London service. It would then be practicable to drop one of the KX-Cambridge stopping services back to Letchworth, as barring the villages these services carry fresh air north of Letchworth - except when the Cambridge-Brighton service is disrupted which is of course a good proportion of the time!