Parents need to know that Little Britain is a very very funny show which 12 year olds and teens will probably want to watch because it is verrry verrry funny.
However, please make sure that your kid understands sexuality and other adult themes, because there is a lot of sex, including some very disrespectful portrayals of transvestites, homosexuals etc. and the whole show deals with very adult themes including transvestites, homosexuality, elderly people, overweight people, asian/black people etc and mercilleslly makes fun of all these issues, and although its funny it is also very disrespectful, so discuss the fact that its not ok to make fun of people who are different just to make sure... there is some nudity (fake but realistic boobs and bums and vaginas and penises), and a good dose of toilet humour (people wet themselves, projectile vomit on others in public etc.), so i think that if your kid is 12 and mature and understands serious sexual topics, can put up wit harsh language, disrespectful humour and racist remarks, go for it!
If you want a more classy, clever and less crude british sketch show, try The Catherine Tate Show, it's much funnier and better quality :)