Last year Conference voted for its NEC to review this policy. The NEC did so and carried out a fact finding mission to Palestine, including meeting up with representatives of its major union, the Palestine General Federation of Trade Unions (PGFTU).
During this visit UNISON representatives believe that they were clearly told that the PGFTU wanted them to not break off all links with Histadrut since there were advantages to the Palestine cause from us having a "critical engagement" policy.
However, the motion was defeated when a number of speakers claimed that the PGFTU wanted us to continue to boycott and made reference to statements made by its leadership which appeared to clearly support that view. This I think caused widespread confusion amongst delegates and led to the motion being lost.
I have never been to Palestine (or Israel for that matter) and I am not in anyway an "expert" in these things but I suspect that the PGFTU do see tactical advantages for them if UNISON had links with Histadrut but they do not wish to do so publicly.
See what happens next year.