Any corporate firewall/router worth more than 20 quid should spot any kind of proxy traffic and turn it into digital poo. Mine does I've also actively blocked anything that doesn't go out on port 80 that ISN'T http - anything trying to hide in port 80 that's not a website gets spat out.

Also bear in mind that if you're attempting to bypass a corporate security mechanism you're at risk of being reprimanded at the very least by your employer. Some of these 'proxy sites' can also use the same mechanism to bypass YOUR security to compromise YOUR network and connected computers.