This could be a much longer post but to keep it simple.
You want to support a London club and you grew up in Biggin Hill. Then perhaps you may want to attach yourself to one of the closest teams to the London Borough of Bromley which looking at the maps seems to be Palace, Charlton or Millwall. And you do say that your family are Palace fans so don't you think there is a connection? You say that you now live in East Sussex so although not actually in London, it is just outside and Brighton and Hove Albion would be your local team too. And looking at how high the Seagulls have been flying this season looking all set for the second division (and who knows, better things afterwards?) and moving in to a nice new stadium, that could be an exciting challenge too. OK, Brighton is just outside London but not too far and after all somewhere that you have far more of a close connection with than Manchester!
And before you start kvetching that none of these sides are in the Premier League - who knows, maybe one day they will be, after all Charlton were a regular Premier team until four years ago and the other teams I have mentioned have all some past experience of being in the top division too. And besides, that is one of the fun things about being a football supporter especially of a team in the second or third tier - the adventure and excitement of mounting a challenege for promotion and the rollercoaster rides that go with it.
So, based on your geographical location and personal history I would narrow it down to Palace, Charlton, Millwall and Brighton. In fact, maybe see if you can get tickets to see any of those teams home games this season or even at the start of next, go and attend them and see which grounds and atmosphere etc you enjoyed the most and then come to a decision.