Lidar observations of volcanic ash are reported, that have been obtained during six flights of the Facility for Airborne Atmospheric Measurements BAe-146 research aircraft over the United Kingdom and the surrounding seas in May 2010, after the eruption of Eyjafjallajökull. Due to safety restrictions, sampling has only been done in areas where forecasted concentrations were smaller than 2000 μg/m3. Aircraft in situ measurements of size-distribution permitted evaluation of a coarse extinction fraction (ranging 0.5–1) and a coarse mode specific extinction (0.6–0.9 m2/g) for each flight. These quantities were then used to convert the lidar-derived aerosol extinction to ash concentration (with an estimated uncertainty of a factor of two). The data highlight the very variable nature of the ash plume in both time and space, with layers 0.5–3 km deep observed between 2 and 8 km above sea level, and featuring an along-track horizontal extent of 85–550 km. Flights on 14–17 May showed typical concentrations of 300–650 μg/m3, and maxima of 800–1900 μg/m3 in relatively small high density patches. Column loads for these flights were typically 0.25–0.5 g/m2 (maxima 0.8–1.3 g/m2). Relatively small regions characterized by a larger ash content have been selected, and the distribution of ash concentrations and column loadings within them proved rather broad, showing how fractal and patchy the observed ash layers are. A visual comparison of our data set with the “dust RGB” maps from SEVIRI showed a good spatial correlation for the larger ash content days. Moreover, ash prediction maps output from the NAME dispersion model show reasonable agreement with the overall magnitude of the observed concentrations; in some cases, however, there are positional errors in the predicted plume location, due to uncertainties in the eruption source details, driving meteorology, and in the model itself.