Surely one reason of rehashing scenes from TV series into movies, is that it appears to be cheaper to repeat the films than the actual TV series.
Before the likes of UK Gold, Granada Plus, ITV 3 etc, when did you see certain sitcoms repeated en masse?
I think it depends on the series - repeats of Dad's Army have been regular fixtures of BBC1/2 ever since it was made, long before the archive channels existed (and isn't it also a regular feature of the 'backup' schedule to air when a live programme goes kaput?). Yet I can't remember the last time the film was shown (awaits the inevitability that it got aired 2 weeks ago!). For other sitcoms the reverse is true (particularly On The Buses, although it's not an example I like to use as I don't believe it's as simple as a BBC/ITV split).
The Dad's Army feature film was aired relatively recently....either late last year, or early this. But the TV series itself has, as has been suggested, been a "regular fixture", especially in more recent years. It's interesting that On the Buses gets fairly regular repeats on ITV3 these days, but other series of similar vintage e.g. Please Sir, Father, Dear Father and And Mother Makes Three are much less in evidence, especially the latter two, which seem to have slipped "below the radar".