Mandatory Palestine was effectively a 3 sided civil war. Groups like the Irgun committed terrorism upon the British Mandate the “Palestine Police” who are colonial cops effectively, because they felt the Mandate was going to create an eventual pluralistic state. The Palestinians and other Arabs began militarizing in response to several massacres by Jewish settlers. Over a hundred British police, many being conscripts stationed there, joined the Arabs or the Zionists

It is difficult to be certain of their number, and reported figures have varied. For the pro-Jewish group of deserters, one article put the number at twelve, while a Jewish veterans’ association estimated that there were around 20. 2 We have identified 17 deserters by name, all of whom joined the Israel Defence Forces (IDF). The pro-Arab deserters were more dispersed, with individuals joining the irregular Palestinian Arab militia al-Jihad al-Muqaddas (Holy War Army), led by Abd al-Qadir al-Husseini and Hasan Salama, as well as the Arab Liberation Army (ALA), a force established by the Arab League, with Fawzi al-Qawuqji as its main commander. 3 Deserters were also witnessed fighting with Transjordan’s professional and effective force, the Arab Legion, commanded by the seconded British officer, John Bagot Glubb.4 A low estimate of this pro-Arab group of deserters was put at 53,5 while the highest estimate set their number at around 200.6 After analysing numerous reports on and mentions of these pro-Arab deserters, a figure of between 100 and 200 spread across all three forces seems correct.

https://www.jstor.org/stable/26987027

Some for very overtly antisemitic reasons though, like those of the British League - Palestine Branch which was an off-shoot of Mosely’s British League of Fascists. https://trove.nla.gov.au/newspaper/article/2739222

You also had professional British soldiers like Glubb and his Arab Legion which officially served Transjordan and fought against the Zionists in the 48 war. Glubb is cool, he and his wife adopted a Bedouin girl and then adopted two Palestinian children orphaned in 1948. His white biological son converted as soon as he was of age, and was part of the Popular Front for the Liberation of Oman, and other leftist Arab-Omani organizations. It is funny how the “going native” stereotype simplifies things in such a way that it can ignore or dismiss British people who genuinely felt a shared humanity with Britain’s subjects/victims.