(Congress Daily 7/20 am)  The White House is threatening to veto the student loan bill currently on the Senate floor, but says it is willing to work with Congress on objectionable provisions.   The Senate bill would cut subsidies to lenders by $18 billion and increase grants to students.   A similar bill that passed the House last week also drew a veto threat.  "The administration supports reducing excess subsidies in the student loan programs and increasing aid to the neediest students," said a Statement of Administration Policy issued Wednesday.