Factual error: The fictional island of Saint Marie where the series takes place is a British territory - after all, the whole gig is about fish-out-of-water British cops in the tropics. Specifically the Commissioner explains the history of the island to DI Richard Poole driving him to the station the first time and says that it was a French territory but in the mid 70s they handed it back to the British. Despite that, throughout the whole series the police and almost every local drive (on the left) vehicles with French registration plates, specifically with the 971 code of Guadeloupe, where the series is shot. In the first seasons the British police also routinely sends evidence and other critical parts of the job to labs in Guadaloupe, effectively out of the country. Seems quite absurd that in 40 years the British wouldn't have established vital parts of administration and rely entirely on French neighbours.
Factual error: Marie is a female name. Therefore, the island should be Sainte Marie, not Saint.
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.