To annex something means to add it to an existing state. Israel by definition can't annex something before it exists.
The UK controlled the area at the time, but they basically gave up and left. I don't understand how you could say they were annexing it at this point in time either.
Israel is closer to being a succesor state to mandatory palestine than anything else. Generally international law recognizes the right for ethnic groups to self-determination (Palestinians often rightly complain that they effectively have not had this in the modern period, but the right applies to both sides). It is not considered annexation for a group from one state to split off and form a new state via self-determination.
To give a comparison, i think your argument would be the equivalent of saying ukraine annexed russia because they unilaterally left soviet union.