Gibraltar.
At least two, depending on how you define 'rock'. Gibraltar is often called 'The Rock'. Britain also owns Rockall, way out in the North Atlantic, and, of course, many rock stacks along the coasts, the best known of which is probably the Old Man Of Hoy.
Gibraltar :D
Note that Rockall (note spelling) and all the stacks are in the ATLANTIC, hundreds of miles from the Mediterranean. Gibraltar is the rock in the Mediterranean owned by the UK.
Gibraltar is called "The Rock" and is British terretory.
That's the 'Rock of Gibraltar'
The Rock of Gibraltar
Gibraltar.
The Rock of Gibraltar
The Rock of Gibraltar
The Rock of Gibraltar
Gibraltar
Gibraltar
Gibraltar
Gibraltar
The "rock" you inquired about is the British self governing territory of Gibraltar. Spain ceded the territory to Britain with the Treaty Of Utrecht in 1713.
The Rock of Gibraltar.
The Rock of Gibraltar
The Rock of Gibraltar is a British Overseas Territory
The Rock of Gibraltar is technically part of Spain but is actually owned by Great Britain (meaning if traveling from Spain into Gibraltar you must have your passport to get in and out). It is in the northern hemisphere.