Only a rough guide, but the latest 2024/25 non-league predictions on non-league maps has us placed in the Isthmian League next season. A lot can change though between now and the end of this season.
docs.google.com/spreadsheets/d/1j9Xsb52BW6OYS98S4SIM4ueejxbA2I9CHePzrzlqSlk/edit#gid=1484938441If it all goes pear-shaped and southern clubs need to be placed in National North again, the candidates (in order of most northern first) would be:
51.8759N - Braintree Town*
51.7740N - Oxford City (potential relegation from National Division)
51.7543N - St Albans City*
51.7539N - Hemel Hempstead Town (outsiders for relegation from National South given their recent dismal results)
51.7532N - Chelmsford City*
51.7509N - Merthyr Town (potential promotion from Southern League Premier South)
51.6991N - Chesham United (potential promotion from Southern League Premier South)
* National South promotion candidate
If Oxford City are relegated and Merthyr Town promoted I'd imagine both may be "persuaded" to head North, despite the latter's location being further south than four Home Counties National South clubs.