Just to add further insight. The only alternative route covers using the Underground in zone 1, so maybe Elephant & Castle to Blackfriars then District to Victoria then East Croydon. As you aren’t doing that you are charged the default route/fare which is only zones 2-5. In terms of the change at Blackfriars, I’d recommend being at the back of the train heading into Blackfriars, use the south of the river subway between platforms as there are no gates and then be at the front of the train for exit at East Croydon.