changing sink with unvented HW System?

I want to change a sink. Old sink was not replaced at the time of installation of unvented hot water system. Have found out that there is no hot water stop cock to the tap. If I turn off the mains cold water tap I understand the pressure will be reduced and theoretically not much water should come out of the tap. However. Will this mean that I'll get air bubbles in the system or any other problems that might arise. Are there any easy ways to do this? As I live on a remote scottish island any help would be greatly appreciated