hmmmm...

Firstly, that documentary came out in 2007 (almost 20 years ago!!!) when there were a few people still trying to deny climate change. Even Boris Johnson (who denied human-caused climate change as late as 2015) has changed his tune more recently (2021) https://www.theguardian.com/commentisfree/2021/sep/21/johnson-climate-denial-delay

Secondly, it wasn't a BBC documentary (which would have given it more of an air of credibility). It was aired on Channel 4 (but not made by them). Also, much of it was disputed at the time (including some of those taking part - who felt they were misrepresented)
https://en.wikipedia.org/wiki/The_Great_Global_Warming_Swindle