Is it true that the Gibraltar Strait used to be territory belonging to the British Empire?

True

Explanation:

Gibraltar was 'the only British colony which was part of the mainland of Europe; it was a tiny peninsula on the southern tip of Spain.

Gibraltar has remained under British control ever since, in spite of various attempts to take it back, including an unsuccessful siege by Spain that lasted for nearly four years until 1783.

this forms a narrow, steep sided inlet (sometimes deeper than 1300 metres) connected to the sea. the terminal moraine pushed down the valley by the glacier is left underwater at the fjord’s entrance, causing the water at the neck of the fjord to be shallower than the main body of the fjord behind it.