I was wondering what county Forest Hill used to be in? (If in one at all!)

I would presume it's not always been in London. The obvious choices would be Kent or Surrey. I am convinced it must have been Surrey as I'm sure Croydon used to be in Surrey once upon a time. My other half reckons it was once in Kent (his logic being that we're in the borough of Lewisham, which is near to Sidcup, which is in Kent... tenuous I feel!)

Please help me settle this dinner table debate! (and please let me be right! !)

Cheerio