On an iPhone, JavaScript should be turned on by default, but if it was disabled at some point, many websites will appear broken in the Safari browser. To enable JavaScript, go into the Settings app on ...

Do I need to enable Javascript on channel4.com and if so, how do I do that?

6. Just to be sure that Javascript is enabled, repeat the previous steps to enable Javascript. Check also if there are apps interfering with Java or your browser.

If JavaScript is turned off, this effectively negates reCAPTCHA's capabilities, hence, the reason to prevent users who intentionally disable JavaScript in their browser.