How can I enter the British National Marathon Championship Event and the Manx National Marathon Championship Event? 

For this you will need to have a valid BC race licence and to be a member of the Manx Mountain Bike Club.  Once you have satisfied these requirements, option 4 on the event entry page (“Manx 100 – Manx 100 miles (including British & Manx Champs)”) will allow you to enter both of these events.