how to make all port 80 http traffic go through isp proxy server

basically im looking for a way to pass all http or port 80 traffic through isp proxy server as some of my clients use the address in IE or Firefox proxy server settings and web pages load quicker with it but some dont add the proxy settings hoping they might be able to bypass proxy but in which case web traffic becomes slower for them inspite of isp using a transparent proxy server, meaning all traffic by default goes through their proxy server but performance is better if proxy server address is specified.
basically pfsense should be able to divert all port 80 http traffic through the isp proxy server specified if specified or not specified in client browser.

i use pfsense embedded 1.2 release on ALIX 2c3.