I think this is a much older thing than you all recognize, and I think it definitely spans cultures, if not "So...", it's "Well...", "umm", "uhh", Spanish speakers say "Pues...", these are just terms people use to give their minds an extra second to think after their mouth has started moving. It eases the transition from thought to speech. Don't know how common it is in the UK and can't remember too much from the few times I've visited.
It is a terrible habit though, I taught a public speaking class once and with practice you can get out of the habit of using them, but its very difficult and takes conscious effort.