Primarily, did the mandate provide some sort of de facto protection against invasion from the Arabs?
But considering that the British had a mandate over Palestine, why did they not intervene in the civil war that followed the UN Partition?
I would rephrase your first question as "Primarily, ... invasion from the Arab countries?"
And the answer for this one is yes. It would be declaring war against the UK, and the UK was still superpower back than.
For the second question, I will reply with a Question:
What reason did you find for the UK to interfere in this war? Because I didn't find any.
The tension, and the violence increased dramatically after UN-proposed partition. Even though war was officially declared by the Arab countries only after the Israeli independence declaration, the situation was pretty rough before.
The UK was in her last days as an Empire. They had made the decision to get out of Palestine, in other words, to runaway from this sick bed. Therefore, interfering in the war will act against this decision.