In this article, we present the first detailed simulation of the antineutrino emissions from an Advanced Gas-cooled Reactor (AGR) core, benchmarked with input data from the UK Hartlepool reactors. An accurate description of the evolution of the antineutrino spectrum of reactor cores is needed to assess the performance of antineutrino-based monitoring concepts for nonproliferation, including estimations of the sensitivity of the antineutrino rate and spectrum to fuel content and reactor thermal power. The antineutrino spectral variation we present, while specific to AGRs, helps provide insight into the likely behavior of other reactor designs that use a similar batch refueling approach, such as those used in RBMK, CANDU and other reactors.