I'll put in a word for Denmark Hill - not more than 15 minutes to any of Victoria, London Bridge and Blackfriars. The good thing is that a significant proportion of the train population are medics trekking in from Sevenoaks to work at Kings, so you find the trains clear out before you get to into town properly. Before 8 o'clock the London Bridge and Victoria trains are both quite bearable and if you're quick you'll probably get a seat. That said, London Bridge trains are only every half hour (24 & 54) but Victoria trains are pretty much every 6 minutes during peak times (53, 56, 06, 15, 21 27 off the top of my head). Plus I find it a generally nicer place to wait in the morning. And they have a coffee shop :)