In this study an alternative ‘top-down’ approach was used to provide direct measurement of the net UK flux of greenhouse gas emissions at the UK scale, using aircraft sampling of the entire plume of GHG emissions downwind of the UK. An atmospheric mass budget over the country was established for each experimental flight and a direct calculation of the UK budget made from concentration measurements upwind and downwind of the UK. The measurements reveal annual UK emissions of CO of 2400 kT yr^-1 and CO₂ of 514 MT yr^-1, within 10% of the NAEI emissions of 2400 kT yr^-1 for CO and 555 MT yr^-1 for CO₂. However, estimates of N₂O (330 kT yr^-1) and CH₄ (3300 kT yr^-1) emissions are significantly larger than inventory estimates of 130 kT yr^-1 and 2400 kT yr^-1 respectively. This suggests that there are potentially important sources of N₂O and CH₄ not currently included in the inventory, that the strength of some sources is underestimated or a combination of the two. The inverse modelling approach further validated the NAEI CO and CO₂ emissions while providing further evidence that the NAEI emissions underestimate the UK emissions for N₂O and CH₄. The mapped Halocarbon emissions produced source locations consistent with localised industrial sources. The oxidation analysis showed oxidation rates tend to be consistent for individual plumes between flights with greater variation between different plumes reflecting the differing nature of the plume sources. The average oxidation rate for NO_x for a power station plume is 1.4 ± 0.66% N hr^-1 compared to 4.4 ± 3.95% N hr^-1 for an urban plume and the average oxidation rate for SO₂ for a power station plume is 4.8 ± 0.76% S hr^-1 compared to 6.5 ± 5.71% S hr^-1 for an urban plume. The average oxidation rate for the whole UK was estimated as 3.2 ± 2.38% N hr^-1 for NO_x and 5.3 ± 1.80% S hr^-1 for SO₂. Uncertainties in the method of measurement are small relative to the magnitude of the differences between the inventory and measurements for individual flights, but extrapolation from the 14 flights to provide annual budgets is an important source of uncertainty for N₂O emissions, which show considerable temporal variability in emissions. These new data provide a viable approach to estimate annual country scale UK emissions to test the quality of the existing inventory and to verify the progress of the emission reductions with time.