Every Asterix full length animation film is fairly fun, but Asterix in Britain is undoubtedly one of the best, along with The Twelve Tasks of Asterix. While children will surely enjoy Asterix in Britain, it can also be enjoyed by adults.
The soundtrack is particularly good (even epic at times), and the animation is very funny. This film is a relatively faithful adaptation of the comic strip, with some pertinent additions, and the jokes are good. I saw it in French and much of the humor stems from the stereotypical speak of the Britons, caricatures of British people, so I don't really know how it would translate in English, but I'm told the translations of the comics tend to be pretty good in capturing the original humor. The authors admired the British very much and they show it here as the Britons are depicted as a brave people, and the little jokes are akin to "love taps" more than anything else, so British people shouldn't be offended by it. There are only little stabs about bad food, tea-drinking, sports-loving and nice lawns.
Anyway, it's a fun little animation which might look a little dated (it was released the year of my birth, in 1986), but fans of the Asterix comics will surely enjoy it.