They probably were telerecordings to film as that was the easiest way to export them at the time, rather than faffing around with converting from PAL to NTSC or SECAM or whatever else is out there, just point a camera at a screen and record that to film. Its probably ironic that its these same recordings that came back to the BBC to replace the ones they'd wiped in the first place.
Dad's Army must be about the most rerun show on terrestrial TV. Seems to have been a regular fixture on BBC2 Saturday nights for a few years now and for many years was as regular on early evening BBC1.
Semi regular, because it disappears when sport/Proms/something else is on to displace it in the schedule. I wouldn't be surprised if its treated more as a schedule filler on a Saturday night.
Its a shame really most of the cast of Dad's Army didn't live to see how much loved it would still be fifty years later. Believe its only Frank Williams and Ian Lavender of the original cast who are still with us.