I use the Penge East service to Farringdon (next stop St Pancras) every morning - I've never seen the 8:04 without a seat available.
If you can't get a seat on the 8:04, wait a couple of stops and lots of school kids (well behaved ones at that) get off at West Dullwich. (I know.)
The 7:39 however is somewhat packed by Penge.
I find it a good service, except for when the driver walked away at city thameslink (his hours were up). The snow didn't help earlier this year either.
I've seen that train at Sydenham Hill whilst waiting for my train to Victoria often enough, and it always appears to have quite a few seats free. Only used it a few times myself though, and generally been fairly good.