TANNIC! I was really puzzled by the tonic thing.
As I understand it (and chemistry majors, feel free to correct my limited chemical understanding) tannic acid, which comes mostly from the grape skins and stems, breaks down over time, allowing more of the fruit flavor to come through. Tannin isn't tart exactly, more bitter. It's the flavor that many people don't like in red wine, but wine lovers weigh as part of a good balanced red. Certain reds have more tannin than others - cabernet sauvignon is typically more tannic than syrah or sangiovese, for example, but a lot of it depends on how the wine is made. Wines with more tannic structure are capable of aging longer than wines with less tannin. Since the US market isn't interested in aged wines, by and large, most reds in the supermarket don't have the structure needed for long term aging.
It's definitely not a British thing, it's part of the winemaking process for any red wine.