An email that has gone out to UCU members draws their attention to the legal advice the union has had:
It would be beyond the Union's powers and unlawful for the Union, directly or indirectly to call for or to implement a boycott by the Union and its members of any kind of Israeli universities and other academic institutions; and that the use of Union funds directly or indirectly to further such a boycott would also be unlawful.You can read the rest of this communication at Engage. See also here.