&source=hp is just giving credit to
Hewlett-Packard for the redirect..
(from now on let's do &source=Hawkee)

You could do simply this:

<form method="get" action="http://www.google.com/search">
<input value="" name="q" />
<input type="hidden" value="en" name="hl" />
<input type="hidden" value="lang_en" name="lr" />
<input type="button" value="Search" onclick="submit()" />
</form>