You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query-continue>
    <allfileusages gafcontinue="BaronTweedmouth.jpg" />
  </query-continue>
  <query>
    <pages>
      <page pageid="28869" ns="6" title="File:BBC StromnessMuseum MarkVIIIDumaresq 1.jpg" />
      <page pageid="28870" ns="6" title="File:BBC StromnessMuseum MarkVIIIDumaresq 2.jpg" />
      <page pageid="28871" ns="6" title="File:BBC StromnessMuseum MarkVIIIDumaresq 3.jpg" />
      <page pageid="6410" ns="6" title="File:BR1534 Plate4.jpg" />
      <page pageid="12331" ns="6" title="File:Backhouse, Oliver, Australia IWM Q 18757.jpg" />
      <page pageid="4209" ns="6" title="File:Backhouse in Lion, National Archives of Canada PA-6543.jpg" />
      <page pageid="12402" ns="6" title="File:Ballard, NPG Ax39039.jpg" />
      <page pageid="5394" ns="6" title="File:Barlow.jpg" />
      <page pageid="31115" ns="6" title="File:BaronCarson.jpg" />
      <page pageid="31093" ns="6" title="File:BaronHampton.jpeg" />
    </pages>
  </query>
</api>