The fare structure from the GN stations is quite interesting and not logical! You pay the same fare to any NR station in zones 1-6 (travelling via z1 and using NR only) as you do to Moorgate, this would include to South Bermondsey. So, for example, Hertford N to East Croydon, Slade Green, Hampton Ct, Elstree & Borehamwood, Hatch End are all £7.20 off peak, 10p more than a journey avoiding zone 1. This fare also covers the tube between Finsbury Park and Kings Cross on the Victoria and Piccadilly lines as well as Kings Cross to Liverpool Street on the Northern (excluding starting / finishing a journey at Angel), Circle , Metropolitan or H&C lines and Farringdon to Abbey Wood (but NOT Stratford and beyond) on the Elizabeth Line. The rationale for a lot of this is that NR tickets have traditionally had interavailability with the underground between Finsbury Park and Moorgate going back to the pre-1975 time when Hertford North trains ran to Moorgate via Kings Cross and the ‘widened lines’.
Any journey requiring a tube journey, or Elizabeth Line to Tottenham Ct Red and the West, beyond those listed above will cost more – £9.90 for any journey ending in zones 1-6. The same applies to C2C, GA, Elizabeth line (Stratford & beyond), and Weaver line stations in z2-6 via Liverpool St (the assumption is you use the tube between Moorgate and Liverpool Street even if you walk via the Elizabeth line platform).