Liquid Air Energy Storage (LAES) is a large-scale storage technology, which is expected to play a key role among competing solutions thanks to its high energy density, long lifespan, flexibility and absence of geographical constraints. However, there is generally little attention on the existing linkage between the power system and energy market services LAES can provide and its thermodynamic performance. Yet, it is not possible to separate these two aspects, as storage profitability is largely influenced by the regulatory assets it is operated on. With the aim of bridging the gap between technology development and its actual integration in the energy grid and incumbent markets, this study attempts to quantify the capability of a 100 MW/300 MWh LAES plant to provide different energy and grid balancing ancillary services for frequency control in the UK market. More specifically, arbitrage, Short Term Operating Reserve (STOR) and Fast Reserve (FR) are considered. A model is developed that is capable of capturing the off-design conditions naturally arising from real system operation. Characteristic maps for rotary machines and off-design model for the heat exchangers are incorporated in a classic thermodynamic framework. Model validation is carried out against results referring to an analogous plant, under design conditions. The numerical tool is then used to simulate LAES operation for the proposed assets. The effect of real operation is projected on thermodynamic variables along the process and finally on the performance and economic indicators for the system. Revenues from each operating mode are estimated and comparisons are drawn. Uniquely in this work, realistic economic performance of LAES is grounded on a solid thermodynamic foundation, allowing assessing the impact of the LAES technological constraints for playing in different grid balancing markets.