I love everything that Monty Python ever did, but I would hardly call Life of Brian the funniest British comedy of all time.
Respectfully, I disagree. I think the author overstates his case. LOB could easily be made today.
Holy Grail reached a cult status that LOB never reached, for instance.
Meaning of Life is an odd duck -- most people didn't like it as much overall but love all the scenes in it. I'd still rank it (a bit) over LOB.
It is probably the surprise effect; it was the first Monty Python I ever saw.
Life of Brian being rated as one of the best British comedies I can understand if only because it had more of a broader appeal (at least more widespread coverage thanks to the religious hullaballoo).
Grail on the other hand, may appeal to the cult market more and be more popular on that front (being the type of people who might vote on IMDB).
I'm grasping at straws here, but at least in my books the religious aspect of Brian (combined with the overall quality of the film) would lead me to say that it's more important than the other two Python films.
It's also telling that the two films you regard as better than Life of Brian are both by the Pythons :)
Maybe without the one scene... (where he wakes up to a crowd of his followers.).