I'm in Balham. I was trying to find my local FA and think it is Surrey, but that means a bit of a trek to go on any courses. It seems the original is only a few hundred pounds so not a massive outlay.. I'm assuming it is a pre-requisite of getting involved with any local teams..
I don't believe you need to have any specific football qualification to coach football. But it's advisable you go and do a course to better yourself and those who will learn from you.
So that's at least the FA Level 1 course, as well as the First Aid certificate.
It gives you the VERY basics, in terms of how to set up an organised session (not much detailed football technical content when I did it). Level 2 had more about it, in terms of understanding footballing principles.
Having said that, the structure and course content of the FA Level 1 and Level 2 course has recently changed, just last August I believe.
Previously, coaching Adults and Kids were different strands on the ladder. Now they're incorporating the Youth Modules into the Level 1-3 courses, so whilst the prices have gone up, you're actually getting more bang (quality content) for you buck (as well as getting qualifications faster).
Also, you're not limited to any county FA. I wanted to progress up the ladder and get my badges quickly so I've travelled around the UK doing courses across a lot of county FAs. It really depends what your ambitions are. The London FA run quite a few of their courses at Douglas Eyre,
E17 7HE. That's an hour on the train. Not too shabby.