In qualification to the above writeup, Rockall is composed of basalt as opposed to granite. Rockall is part of the Giant's Causeway/Isle of Skye continental flood basalt province, that stretches to Greenland.
This formed in the rifting of the North Atlantic when Europe separated from Greenland and Canada under the influence of the mantle plume currently found under Iceland. This mantle plume has remained roughly in place, thus causing a hot-spot island to form at an oceanic ridge, which is why Iceland is so much bigger than Hawaii. This event occured 59 million years ago, at the beginning of the Tertiary period. Geologically speaking therefore, Britain has far less justification for owning this speck than Iceland. However, this didn't prevent it playing a key role in the Cod Wars of the 1950s-1970s, one of Britain's most surreal foreign policy adventures. One day Britain will stop having bizarre wars over territories acquired/influenced in the days of empire (Suez, Falklands, Iraq), but not today.