The fares from Hertford North seem to be a bit muddled. The same default fares are charged for travel to Camden Road, Blackfriars NR and East Croydon. It doesn’t seem to matter whether you avoid zone 1 or not, as long as you don’t change to the Underground. It might be worth asking GTR why this is.
Also, even if LO take over the Moorgate services don’t expect fares to change beyond zone 6.