I think there's some confusion here. As far as I'm aware you should be covered on your insurance policy for missing your flight but I don't think they'll cover the full cost of a new flight.
If you had booked the whole trip through a travel agent it would be up to them to arrange to get you home at their expense, however I suspect as with most of us you booked the flights independently.
Is Johncar54 trying to say that if I missed my flight from heathrow due to the M4 being closed and all the alternative roads being grid locked then my travel insurance wouldn't cover me for a missed departure? What if I booked through an airline that offers a chauffeur service to the airport, does their responsibility start when they pick me up, what if their car breaks down?