I am interested in this as I am the maintainer of the BTEC grading plugin
I am fairly sure there is no such admin setting for the assignment type. I considered doing some kind of Assignment clone in the past that was identical except defaulting to the btec grading method and scale but decided it would be a bad idea.
It would be possible and not especially complex to create a new admin setting and then get it pulled into each new instance of the assignment. The settings code would go into the existing settings.php file with a new value something like
$name = new lang_string('gradetype', 'mod_assign');
$description = new lang_string('gradetype_help', 'mod_assign');
$setting = new admin_setting_configduration('assign/gradetype',
That is not correct or tested, just an indicator of what would be required (you would need another for the grading method. However then you a patched Moodle that would require updating with each new version of Moodle. Could add 20 to 30 minutes extra fiddling time to each upgrade