I don't know about the UK specifically but I think that circumcision became so popular in other parts of the world like the US, Australia/ NZ because it was the parents 'showing off' that they had their little boy in a hospital rather than a home birth. It was thought that only the more well off families could have a hospital birth.
The widespread adoption of male circumcision in the United States began with circumcision policies implemented in the US military. Army and Navy physicians believed that the medical evidence available at the time was sufficient to justify uniform policies requiring draftees and servicemen to undergo the procedure. Although circumcision remained the standard treatment for phimosis, military surgeons also began routinely circumcising draftees and servicemen for health and hygienic reasons. They believed that circumcised men were less likely to contract syphilis or gonorrhea while off duty, and less likely to develop balanitis or posthitis when deployed to regions where maintaining proper personal hygiene was difficult.
Most Army and Navy surgeons used a forceps-guided method in which the foreskin was first drawn forward, clamped with forceps, and then excised. Many men feared the procedure because of the pain often associated with it, due to inadequate analgesia. However, following the invention of Novocain and its adoption for minor surgical procedures, many tolerated the procedure quite well, especially when it was preceded by an intravenous morphine injection to reduce anxiety. Preoperative preparation remained painful though, as surgeons continued to use carbolic acid or mercuric iodide to disinfect the glans. One report described a Navy surgeon who circumcised 40 servicemen in a single day, operating on groups of four at a time because the infirmary contained four operating tables and 40 beds. He could have performed even more procedures had additional facilities been available.
Regardless of the advances in analgesia and anesthesia, many men remembered their circumcisions as traumatic experiences. As a result, some requested that physicians circumcise their sons early in life in order to spare them the ordeal of undergoing the procedure after being drafted. The invention of the Gomco clamp in the mid-1930s was, in part, a response to the growing demand among American fathers to have their sons circumcised. The device gained popularity not only in the United States but also in Canada and Australia, as it was marketed to hospitals and private physicians as a bloodless circumcision instrument. It remains one of the preferred methods of circumcision today.