I am never sure which of the two rail threads is the right one but....
I have now used the Oyster 'Pay as you go' on the train from Forest Hill to London Bridge for two weeks and have to say it is brilliant. It only costs me £2.60 to get to London Bridge and £2 to get back. Because I don't do the yourney every day (only 4/5) it costs about the same as a monthly london terminals rail card and can cope with the fact that sometimes I make entirely different journeys.