Image from SEVERI (Eumetsat) at 18GMT (8 PM Euro summer time). The combination of two visible channels and one IR channel gives the bluish tint to the clouds.

The red arrow is pointing at the overshooting top on the thunderstorm, indicating a very strong updraft. The BBC says 4 were killed when a stage collapsed at a music festival. Surface weather obs (from just before the storm developed) indicated temps near 27℃ (80℉) and dew-points near 18℃ (64℉). A surface front was moving into Belgium as a strong upper level trough pushed through the UK, and this set the stage for severe storms in a place that sees few of them.