Or just order some walkthrough, dual voltage units with low density seating (similar to 455s) and run it in with FCC, install ATP in the form of ETCS between Finny Park and Moorgate, and as part of the upgrades of Finsbury Park provide the Moorgate Lines dedicated platforms to changeover between ATP/ATO and AWS/TPWS Manual.
This should be enough to be able to step Moorgate up to 12tph with 6tph to WGC and 6tph via the Loop in the peaks. A reversing siding immediately north of Finsbury Park or a station where it is possible to reverse can be added to provide 12tph throughout the day between Finsbury Park and Moorgate, for connections to Thameslink.
I'll do some more thinking on this later tonight when I'm more awake...
That actually sounds good although may I make a suggestion

There is already a Reversing Siding at Bowes Park so why not extend the proposed 12tph between Moorgate and Finsbury Park to Alexandra Palace then run ECS to Bowes Park to reverse then restart the next service from Alexandra Palace.
OR the other alternative I would going to suggest.....
Moorgate to Gordon Hill as 4tph
Moorgate to Hertford North as 1tph vice 2tph
Moorgate to Stevenage as 2tph vice 1tph (Limited Stops)
Moorgate to Welwyn Garden City as 5tph vice 2tph/3tph
OR the third way....
Moorgate to Gordon Hill as 4tph
Moorgate to Stevenage as 2tph vice 1tph (semi fast)
Moorgate to Welwyn Garden City as 6tph vice 2tph/3tph
Now as to the rolling stock, anyone who replaces the 313s with a 378/S Stock Met layout is great
Hertford North and Welwyn Garden City - Moorgate services are all DOO, and there's never a member of staff on stations like Essex Road either :P
Umm you're quite wrong in saying there are no staff at Essex Road - it's a safety requirement that staff are provided at Highbury & Islington, Essex Road, Old Street and Moorgate simply because of the location of the platforms for these stations.
They're there not to dispatch trains except in the case of Moorgate but rather to act there in a safety role ie evacuate the platforms in a emergency or manage disruption.