These half hour shows just never transfer to the full movie format even when the half hour is quite good
This won't, because the source material is about as funny as a flick in the baws, but there have been successes over the years: Dad's Army, The Fugitive, Mission Impossible (some of them), Naked Gun (all of them), South Park: Bigger, Longer, Uncut, The Untouchables, Serenity... There are probably more!
Saturday Night Live created characters which turned into great movies too, like Blues Brothers, Coneheads and Waynes World.
Unfortunately, Mrs Brown is not a character to make a great movie out of. I don't normally complain about the Beeb spunking money on sh!te, but this is extracting the urine big time.