I don't really know what you are arguing for or against here, a sequel could cost twice as much and take in twice as little and still be more profitable than a 6 episode series. I agreed that the third series and film weren't good except the odd chuckle, but that didn't stop them getting their best ratings, and people going to the movie and coming back to buy the DVD/Blu in huge numbers too.
Also, any films profits would equal a bigger budget for Film4 for future films, so yeah, another plus there. Always good to see British productions out there.
I just think that another film would be a bit of a gamble give how ordinary the first one was. I for one wouldn't pay cash to see it. And unless there is a massive quality improvement the 3rd and subsequent installment would probably be marginally profitable (at best) straight to video offerings. British comedy series converted to film is rarely a satisfying experience.