The trick is to buy an online ticket from the nearest station to EC. You Advance ticket is valid while you are on the train and the other is valid for exiting. OK it is an extra 60p or so for peace of mind.Which is kind of annoying if you need to get off at East Croydon.
It is just that ticket pricing has nothing to do with the journey. I did London to Aberystwyth regulary. You change in Birmingham. The thru price until this month was LESS than EITHER the London to Birmingham OR Birmingham to Aberystwyth. Cheaper to travel ether leg with a through ticket. As long as you have a valid ticket to exit you might as well cheat them as they cheat you.
Funnily enough the same is true for National Express Coaches on the same route.