Flexibility, particularly in terms of reserve services, is an essential requirement of power systems with high penetration of renewable electrical generation, which can reduce undesirable curtailment and enable higher integration of clean electrical power from renewable generation. Reserve services are related to additional active power sources available to the grid operator in the form of either increased generation or demand reduction. There is increasing evidence that commercial buildings can provide such reserves. In this paper we present a Model Predictive Control approach to optimization of flexibility afforded by commercial buildings for the provision of Short-Term Operating Reserve in the United Kingdom without compromising the comfort of the occupants. In this reserve scheme the flexibility is to be made available only during selected hours of the day and is to be provided for a few hours with a slow response time (<= 5 minutes) if required by the Transmission System Operator, National Grid. Simulation results show that a commercial building can provide Short-Term Operating Reserve and yield an economic benefit in a robust manner, without violating the indoor comfort of occupants.