We may be able to get 'out' of Forest Hill every 7 minutes but that doesn't equate to a similarly frequent return journey.

You need to make a choice of either South Eastern train from London Bridge or Overground from another station. London Bridge service is every half hour, which is appalling for a well-used line. Overground is more frequent but not necessarily as convenient or quick after allowing for the interchange, eg if using the infrequent Hammersmith & City line to connect at Whitechapel.