I've been battling on this one for months and years. I got a definitive response from TfL that Canada Water is NOT regarded as a permissible route. Both Lewisham Council's head of transport and I have asked TfL to reconsider this decision.

Waterloo, Charing Cross, Southwark and London Bridge underground station barriers will accept point-to-point tickets from mid-January, when interchange is not permitted. But I'm told that starting at Forest Hill and traveling to Waterloo underground via Canada Water is not acceptable, instead I must travel via London Bridge and use that underground station. If you miss the 0823 that means a wait until 0844 for the next LB train and then a crush on the Jubilee line (if you can enter the station).

Obviously I would much prefer to jump on the first train at Forest Hill without caring about the destination, knowing that I can change to the Jubilee Line. I'm trying to work out my options for next year:
1) Obey the rules and travel through London Bridge and the horribly crowded entrance to the underground
2) Buy a travel card for an extra £400 for the year (I'm sure I can think of better ways to spend £400)
3) Ignore the rules and take whatever train I want. I expect the ticket barriers will let me through, so I'll be okay as long as a ticket inspector doesn't stop me between NXG and LB on the underground.

I'm very tempted by the third option and believe that I would have a good case in court to claim that I was not abusing the system. I was entitled to use both of the trains and my start and destinations were valid. Perhaps I just forgot to get off the train at the appropriate interchange?