<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg xmlns="http://www.w3.org/2000/svg" version="1.1" viewBox="0 0 60 30" width="1200" height="600">
<clipPath id="t">
  <path d="M30,15 h30 v15 z v15 h-30 z h-30 v-15 z v-15 h30 z"/>
</clipPath>
<path d="M0,0 v30 h60 v-30 z" fill="#00247d"/>
<path d="M0,0 L60,30 M60,0 L0,30" stroke="#fff" stroke-width="6"/>
<path d="M0,0 L60,30 M60,0 L0,30" clip-path="url(#t)" stroke="#cf142b" stroke-width="4"/>
<path d="M30,0 v30 M0,15 h60" stroke="#fff" stroke-width="10"/>
<path d="M30,0 v30 M0,15 h60" stroke="#cf142b" stroke-width="6"/>
</svg>

Optimize Union Jack SVG with SVGO
Optimize Union Jack SVG with SVGO

The SVG now weighs 259 bytes only (27.45% saving).