From 1922 to 1973 the BBC was the only organisation licensed to run radio stations in the UK. During this period many stations broadcast to the UK using transmitters located outside of UK territory.
During the 1930s a number of continental European stations broadcast English language commercial radio programmes to the UK. The most successful of these were Radio Normandie broadcasting on Medium Wave from Fécamp on the north coast of France and Radio Luxembourg broadcasting on Long Wave from the Grand Duchy of Luxembourg.
All the continental stations closed down in 1939 with the outbreak of World War 2. The Nazis used the transmitters of Reichsender Hamburg among others to send propaganda News & Entertainment programmes by the likes of Lord Haw-Haw and Charlie & his Orchestra to the UK audience.
In the 1950s Radio Luxembourg, the only continental station to resume English language broadcasts following World War 2, moved their English service from Long Wave to 208m Medium Wave. They stayed on this wavelength until the early 1990s.
By the early 1960s neither the BBC nor Radio Luxembourg had quite got the hang of Rock'n'Roll music. The BBC Light Programme was restricted to playing only a few hours of records per week & filled the gaps with bands of session musicians performing their own versions of the hits of the day, in between the Soap Operas, Quizzes & Comedy Shows. Radio Luxembourg's programmes consisted almost entirely of records. Most programmes were sponsored by record companies who wanted exposure for the maximum number of new releases so would cram as many records as possible into each show by playing only the first 20 or 30 seconds of each one.
Since 1958 Several European stations had taken to broadcasting from ships anchored outside territorial waters - thus avoiding local broadcasting laws. Radio Mercur, Radio Nord & Radio Syd broadcast to Denmark & Sweden, Radio Antwerpen was off the coast of Belgium, Radio Veronica & Radio Noordzee were off the Dutch coast.

This idea reached England in 1964 when Radio Caroline, located on a ship anchored outside territorial waters off the coast of Essex, started broadcasting to London & the South East. Caroline was an immediate success & quickly opened a sister station, anchored off the Isle of Man, to cover Liverpool, Manchester, Dublin & Belfast.
Meanwhile in S.E. England, Caroline was joined on the air - and soon beaten in the ratings - by Radio London, whose ship was anchored off the Essex coast within sight of the Caroline ship.
Pop music was not the only thing available from these Pirate stations. Radio 390 broadcast light classical & jazz mixed in with Mantovani-style strings from a former World War 2 gun tower in the Thames estuary off the north coast of Kent. Radio Scotland broadcast a mixture of Top 40 & ceilidh music from a former lightship anchored in the Firth of Clyde.
In May 1966 two new stations, Britain Radio & Radio England started broadcasting from the same ship off the Essex coast. Radio England closed in November 1966 due to lack of advertising, Britain Radio lasted until February 1967 when the ship's antenna collapsed during a storm.
In August 1967 the UK Government introduced a law banning UK subjects from having anything to do with offshore broadcasters. All the stations closed except Radio Caroline. The Final Closedown of Radio London was on the 14th of August.
Radio Caroline limped on until March 1968 when both ships were impounded by a Dutch company owed money by the Caroline organisation.
Radio Caroline returned to the air in 1972 with 1 ship broadcasting to London & the South East, the ship sank in 1980 but Caroline returned once again in 1983 on a new ship & was last heard of from the high seas in 1991.

The tradition of broadcasting to the UK from other places is carried on today by TeamTalk  252, on Long Wave from the Irish Republic.