This is one way to do it. Certainly others will show you different ways to skin this cat. They all will do pretty much the same thing, just in different ways. It'll be up to you to decide which one fits with your whole script better.
#include <String.au3>
Local $file = FileOpen("AR123.html", 0), _
$data = FileRead($file), _
$found = _StringBetween($data, 'SR_NUMBER: ', '<br>')
MsgBox(0, '', $found[0])