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|2158" />
  </query-continue>
  <query>
    <pages>
      <page pageid="2158" ns="0" title="First Lord of the Admiralty" />
      <page pageid="2393" ns="0" title="George Alexander Ballard" />
      <page pageid="2740" ns="0" title="Charles James Barlow" />
      <page pageid="3142" ns="0" title="Oliver Backhouse" />
      <page pageid="3406" ns="0" title="Roger Roland Charles Backhouse" />
      <page pageid="3756" ns="0" title="Dumaresq" />
      <page pageid="3877" ns="0" title="Vickers Range Clock" />
    </pages>
  </query>
</api>