The last time I went to this was over 15 years so can't vouch for it now. Back then it was pretty big and looks to have grown. They claim around 3,000 classics on display. Probably a bit of a mixed bag, but just thought I'd flag it up.

http://www.bromleypageant.co.uk/

Like Cyborg7 I used to go to this when I lived in S London but haven't been in 15 years. Was always interesting and well-supported. Not sure I can go this year on that date.