The Grapple-Orange Herald atmospheric nuclear test of 31 May 1957 on Malden Island, reported by Universal International Newsreel as "British H-Bomb Fired As Debate On Atom Test Ban Rages" on 3 June 1957. It was claimed at the time to be the first British test of a H-bomb. It was later revealed to be a fusion boosted fission nuclear weapon test, where the fusion boosting failed to increase the yield. It still yielding 720 kT of explosive power, probably the largest A-bomb test ever.
Universal International Newsreel
1957-06-03
<a rel="nofollow" class="external free" href="https://archive.org/details/1957-06-03_British_H-Bomb">https://archive.org/details/1957-06-03_British_H-Bomb</a> (first 48s of newsreel)