Good on you man.
You were prepared to stand up for the taxi driver and did!
I watched a documentary on cable a couple of days ago, where they staged a fight in a street in England and a guy was beating a guy with a stick and kicking him on the ground and only 1 out 20 people intervened.
If that taxi driver was my Dad and I wasn't there I wld be glad that you were. And even though it wasn't my Dad I still think it rocks that you put yourself in danger against two attackers who were armed to protect him.
And as other gfy-ers have said use the Good Samaritan defence.
Good luck. Cops can't be everywhere and that's what these attacking pricks know, what they didn't expect was someone like you to be there. Hope the taxi driver is ok too.