Future power systems will have to integrate large amounts of wind and solar generation to drastically reduce CO2 emissions. Achieving this goal comes at the cost of a reduced level of the system inertia and an increased need for fast response services. Previous research has shown the effectiveness of thermostatically controlled loads (TCLs) providing frequency response, and the ability to accurately control the aggregate power consumption of TCLs. In this paper, we explore the design space of frequency response patterns of flexible TCLs. Two distinct frequency response implementations are presented. The first makes the TCLs' power consumption a linear function of system frequency and/or its rate of change; in the second, TCLs respond to a frequency event tracking a pre-programmed reference power profile. Computer simulations illustrate strengths and weaknesses of the proposed implementations in the context of the GB 2020 Gone Green scenario.