If you have APPR mode selected (ie LOC and G/S) and have a valid ILS signal it will not disconnect, the aircraft will autoland as long as at least one autopilot is engaged. Don't be fooled by the CAT 1/2/3 status shown on the FMA, for instance if you had one A/P engaged and one ILS receiver inoperative the FMA would annunciate CAT 1 but the aircraft will still autoland. You are quite correct about the 160ft rule. If you have only CAT1 shown on the FMA you should not use the A/P below 160ft but it will still perform an autoland rather than disconnecting.