Best answer by woz
There obviously is a valve in gas meters or pay as you go wouldn't be possible, and yes there is a relay in Smart electric meters.
They are designed to fail safe, although that's not 100% certain as I suppose it depends on the failure.
As far as hacking goes, I don't think there's been a reported case yet, I think the hackers are more interested in your bank account than your gas (or electric) meter, but never say never...
If it ain't broke don't fix it...
Are smart meters fitted with an internal relay which if it fails, or the meter is hacked, could disconnect my electricity supply ?