What is the difference between doing what you suggest and simply add an ISP's smtp server address in the 'relayhost =' in the /etc/postfix/main.cf file?
Right, I forgot that one...
You have to set the relayhost variable in /etc/postfix/main.cf:
postconf -e 'relayhost = smtp.ntlworld.com'
And don't forget to restart Postfix afterwards.