Julius Caesar has invaded Britain and succeeded in his conquest; but a single village in Kent remains independent. One member of the village, Anticlimax, is dispatched to Gaul to enlist the help of Getafix the druid in providing magic potion for the British rebels. It is decided that Asterix (Anticlimax's second cousin twice removed) and Obelix should accompany him, to help transport a barrel of the potion; but while beating up a Roman galley in the British channel, Obelix mentions the mission, which is reported to the Roman high command in Britain.
In Britain, the barrel of potion is confiscated from a pub cellar, along with all the barreled "warm beer" (bitter) and wine in Londinium, by the Romans, who set about tasting the barrels to find the right one. Soon the whole unit assigned to the testing is hopelessly drunk; whereupon Asterix and Obelix steal all the barrels labelled with Dipsomaniax's name, but Obelix is himself drunk and starts a fight with some passing Roman soldiers. During the fracas, a thief steals the cart with the barrels. Anticlimax and Asterix leave Obelix at Dipsomaniax's pub to sleep off his hangover; but while Anticlimax and Asterix go in search of the thief, the Romans capture the sleeping Obelix and Dipsomaniax, and raze the pub. In the Tower of Londinium, Obelix wakes up and frees himself and Dipsomaniax out of the jail, and the three heroes, after a search, find the potion in use as a pick-me-up for a Rugby team. After this team wins their game, the protagonist seize the potion and escape on the Thames river, where the Romans destroy the barrel and release the potion into the water. At the independent village, Asterix eases the Britons' disappointment by feigning to remake the potion, with herbs later revealed to be tea. With a psychological boost, the village prevails against the Romans, and Asterix and Obelix return home to celebrate.
The chief of Anticlimax's tribe is a caricature of Harold Wilson. The Beatles also make a cameo appearance as bards.
Although many books in the series deal with other European peoples, Britain is one of only two where the book starts with a note from the authors that they are not looking to insult their famous rivals (The English) but to merely make fun of the common stereotypes. The other book given similar treatment is Corsica.
In Anthea Bell's and Derek Hockridge's English translation, the linguistic difference between the Gauls and the Britons is shown by some of the Britons (especially Anticlimax) talking exclusively in stereotypical "upper class" English, including expression such as "This is a jolly rum thing, eh, what?" and "I say, rather, old fruit". In particular, Anticlimax's frequent use of "what?" makes Obelix ask "What do you keep saying what for?"
The book was adapted into an animated film of the same name, which was released in 1986. The adaptation is similar to the book (the main difference being that Dogmatix accompanies his master to Britain).