Get the overground from London Bridge to Charlton, that's just round the corner from the Valley
I think that is how I did it last time.
Last time I did Reading -> Padders -> Tube to Waterloo -> short walk to Waterloo East -> Charlton. Not too bad.
Last time I went there was the 2-1 League Cup loss, where we witnessed a Bongani Khumalo masterclass
You're both wrong; the way to get to Charlton is RDG -> Paddo -> tube Covent Garden [beers] -> walk to Charing Cross -> overground to Charlton