Hello Anabela,

For decrease header space i think you may not need to change into external layout.

you can decrease Top Margin (mm) and header_spacing from Paper Formate as per your need. from "Settings >> Technical >> Reports >> Paper Format"

And if you want to set-up by code then create new paper format as per your need and link with your report.

Example:

For create New paper Format:

<record id="paperformat_custom" model="report.paperformat">

    <field name="name">European A4</field>

     <field name="default" eval="True" />

     <field name="format">A4</field>

     <field name="page_height">0</field>

     <field name="page_width">0</field>

     <field name="orientation">Portrait</field>

     <field name="margin_top">40</field>

     <field name="margin_bottom">23</field>

     <field name="margin_left">7</field>

     <field name="margin_right">7</field>

     <field name="header_line" eval="False" />

     <field name="header_spacing">35</field>

     <field name="dpi">90</field>

</record>

For link-up existing paper format with report:

<record id="<report_id>" model="ir.actions.report.xml">

     <field name="paperformat_id" ref="paperformat_custom"/>

</record>

Hope this helps.