The proportion of the Union Jack is 1:2 and was adopted 1801:

blue = RGBColor["#00247D"];
red = RGBColor[
  "#CF142B"]; Graphics[{EdgeForm[{Black, Thickness[Large]}], {blue, 
   Polygon[{{0, (3 Sqrt[5])/2}, {0, 10}, {2 (10 - (3 Sqrt[5])/2), 
      10}}], Polygon[{{3 Sqrt[5], 0}, {25, 0}, {25, 
      1/2 (25 - 3 Sqrt[5])}}], 
   Polygon[{{35, 0}, {60 - 3 Sqrt[5], 0}, {35, 
      1/2 (25 - 3 Sqrt[5])}}], 
   Polygon[{{60, (3 Sqrt[5])/2}, {60, 
      10}, {60 - 2 (10 - (3 Sqrt[5])/2), 10}}], 
   Polygon[{{0, 20}, {0, 30 - (3 Sqrt[5])/2}, {2 (10 - (3 Sqrt[5])/2),
       20}}], Polygon[{{3 Sqrt[5], 30}, {25, 30}, {25, 
      30 + 1/2 (-25 + 3 Sqrt[5])}}], 
   Polygon[{{35, 30}, {60 - 3 Sqrt[5], 30}, {35, 
      30 + 1/2 (-25 + 3 Sqrt[5])}}], 
   Polygon[{{60, 20}, {60, 
      30 - (3 Sqrt[5])/2}, {60 - 2 (10 - (3 Sqrt[5])/2), 20}}]}, {red,
    Polygon[{{0, 0}, {3 Sqrt[5] + 2 (10 - (3 Sqrt[5])/2), 
      10}, {5 Sqrt[5] + 2 (10 - (3 Sqrt[5])/2), 10}, {2 Sqrt[5], 0}}],
    Polygon[{{60, 0}, {60, Sqrt[5]}, {60 - 2 (10 - Sqrt[5]), 
      10}, {60 - 2 Sqrt[5] - 2 (10 - Sqrt[5]), 10}}], 
   Polygon[{{0, 30 - Sqrt[5]}, {0, 30}, {20, 20}, {20 - 2 Sqrt[5], 
      20}, {20 - 2 Sqrt[5], 20}}], 
   Polygon[{{60, 30}, {60 - 2 Sqrt[5], 30}, {40 - 2 Sqrt[5], 20}, {40,
       20}}]}, {EdgeForm[], White, 
   Polygon[{{25, 0}, {27, 0}, {27, 12}, {0, 12}, {0, 10}, {25, 10}}], 
   Polygon[{{33, 30}, {35, 30}, {35, 20}, {60, 20}, {60, 18}, {33, 
      18}}]}, {red, 
   Polygon[{{27, 0}, {27, 12}, {0, 12}, {0, 18}, {27, 18}, {27, 
      30}, {33, 30}, {33, 18}, {60, 18}, {60, 12}, {33, 12}, {33, 
      0}}]}}, PlotRangePadding -> None]

Here's the result:

The war flag has proportion 3:5:

Proportion 1 would look like this:

Once the flag is constructed, it can be manipulated:

and even wilder: