Do I need to prepare my boiler for the water being shut off at mains?

Yes, I think it would be a good idea to shut the main water supply valve off to the hot water heating system during that time. You are likely to loose water pressure to the house during that time. The heat system should work fine during that time. Then when you know that water pressure has been reestablished, open several of he faucets throughout the house, allow them to run long enough to dispel all of the air from the house piping, 2 to 3 minutes, then reopen the valve to the heating system. For the water heater I would shut that off as well. You will have to open this valve again before doing the air flush. Happy heating, P.