David_Jay,
Firstly, I know that you did mention two Jewish groups who used terrorist methods against the British Administration. If I gave any impression otherwise, then I apologise for that.
In most if not all guerrilla type conflicts (sometimes also described as "brushfire wars, "irregular wars or "low intensity wars") things are often not altogether black-and-white. This is certainly the case with the changing loyalties that you describe in post-WWII Palestine.
By its very nature, a guerrilla type campaign causes major disruption out of all proportion to the number of fighters involved - particularly when carried out in an urban setting. For example, during "The Troubles" in Northern Ireland between 1969 and 1997, the IRA fielded only about 700 fighters. Likewise, during the Malayan Emergency, the British army calculated that it required ten soldiers to be deployed on the peninsula for every one guerrilla fighter the Communists had operating in the jungle. At an earlier time in South Africa, a mere handful of Boer commandos ran ragged the 400,000 British and Imperial soldiers deployed against them during the Anglo-Boer War (1899 - 1902). One could go on and on about the disparity in numbers during guerrilla wars !
The story was much the same in the Palestine Mandate. 100, 000 British troops (i.e. one for every five of the Jewish population) could not contain a terrorist campaign being waged by only a small number of Jewish fighters. (Certainly, as you say, by no means were all the Jewish residents were terrorists!) The results were, though, that very quickly the police were bottled up inside their forts; the army hardly dared venture outside its fortified camps, and the roads in between were dangerous to use because of the prevalence what we now know of as "Improvised Explosive Devices" (IEDs).
A Britain that was by then practically bankrupt from its efforts in fighting a World War simply could not afford to keep up such a fight. (That, too, is another strategy of guerrilla warfare; make the conflict such a long, drawn out, bloody and costly affair as to be ultimately not worth the effort).
My case, though, is that Britain had already decided to evacuate its forces from the Palestine Mandate before the civil war broke out. Britain announced this decision in September of 1947, whereas civil war broke out at the end of November that same year.
PS: My knowledge of Middle East history in more modern times does exceed by a margin my knowledge of Bar Kokhba's rebellion - mainly because I obtained it from sources other than the WTS!