Little Britain is a very funny sketch show but it is nowhere near as well observed as Catherine's masterpiece. Compare Vicky Pollard and Lauren. Many assume that Lauren is Catherine's attempt to imitate and cash in on Lucas's Pollard. But the character of Lauren was invented long before that of Pollard and is a far richer observation of today's inarticulate, low-aspiring youth. Pollard is a much more surreal character, someone who wouldn't be of place in Royston Vasey. Lauren however is scarily real.
Also the 'Nan' is possibly the second funniest 'Nan' creation in recent times, second only to the slightly more sublimity that is 'Nanna' in The Royle Family.
In fact, the majority of sketches on the Catherine Tate Show are very strong. It is just that people assume that a show that has so many different sketches with such a limited number of performers will be stale, but it's not. In fact, it's fantastic.