The United Kingdom held a national preselection to choose the song that would go to the Eurovision Song Contest 1970. It was held on 7 March 1970 and presented by Cliff Richard. The votes were open to the public by post, with the results announced on 14 March, just a week before the Eurovision final itself. Welsh singer Mary Hopkin had been chosen as the UK representative back in August 1969, having made her name with his such as "Those Were the Days" and "Goodbye".

"Knock, Knock Who's There?" won the national preselection and went on to come 2nd in the contest. The song was released as a single and became a hit across Europe, peaking at No.2 in the UK. Belatedly, it also became a minor US hit for her in 1972.

At the Eurovision final, BBC 1 broadcast the Contest with David Gell providing the television commentary, Tony Brandon provided the radio commentary on BBC Radio 1. The contest was also broadcast on British Forces Radio with commentary provided by John Russell.

At the Song for Europe final, Hopkin was backed by singers John Evans and Brian Bennett, with the orchestra conducted alternately by John Cameron and Johnny Arthey. Hopkin was allowed to select one song for the contest, this being "You've Everything You Need", while the other five were selected from over 200 songs put forward by songwriters. "You've Everything You Need" caused a minor scandal on the day of the contest when the Daily Mirror reported that the publishers of that song had told all their staff to send in multiple votes. Whether votes had to be then adjusted by the BBC to avoid a scandal is unknown, but the song ended up being placed second-last.

"Knock Knock Who's There" ultimately became the easy victor, a song written by Geoff Stephens and John Carter. Hopkin recorded all six entries, with the top two placed being released as a single. Of the others, only "Can I Believe" was ever released commercially.

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