Hillstop,
That's a great route via Peckham Rye and you also have the 363 to Horniman and 63 to Forest Hill Road/
I hope I don't get to try it out soon!

I think the P12 takes such a convoluted route that you might be better walking (I did this once as far as Crofton Park).

The other route I tried on Saturday was the train from New Cross to Catford Bridge and then the 185. But that was a rather long journey, mainly spent waiting for trains and buses.

I think the Peckham Rye route from Surrey Quays will take around 45 minutes to the centre of Forest Hill.
The 172 route is likely to take about the same, although I think I did it slightly faster yesterday using the P4.