1940s (World War II). Probably after June, 1941.
Weldons was the British equivalent of McCalls, producing both a women's magazine and home sewing patterns.
Weldon's "So-Easy" line of patterns seems to have started in the late 1930s, and may have been expanded during the war years. According to the web site for the Imperial War Museum, clothing rationing was imposed on June 1, 1941. Utility clothing, which regulated fabric, trims, and findings, was introduced in 1942.
I haven't yet found any indication that sewing patterns were rationed. When Weldons indicates that their So-Easy patterns are "special coupon value designs," I take this to mean that the designs accounted for rationing of yard goods, which did require coupons.
The use of the term "overall" is a shortening of the earlier term "overall apron." ("Overall" is also used in Britain for the sleeveless double-fronted apron that we know here in the states as a Hoover apron or Hooverette.)
The overall would have been important to women is several ways. First, women who did factory work would often have been required to provide their own "work" clothes, and some women probably made their own.
This woman's cheery garment is an overall, the short sleeves not quite covering the blouse or dress sleeve underneath:
Second, with strict rationing reducing one's clothes purchases to about one outfit per year, an overall worn over one's dress or skirt and blouse would have kept them clean and lasting longer.
Here's a lovely photograph of ladies of the Women's Institute in their aprons and overalls, making fruit preserves of some kind (my money is on marmalade.)
Florals seem to have been the most popular print for overalls, and overalls even make their way into books. Chapter 10 of Angela Thirkell's 1940 book Cheerfulness Breaks In starts with:
"...Lydia Keith...went off on foot to Northbridge village with a large flowered overall in a bag."Lydia wears her overall while cooking lunch for evacuees.
The Weldons overall is essentially a simple, button-front shirt waist dress. Raglan sleeves would have been a little simpler to make up than set-in sleeves.
This unprinted pattern is unused.