All details in a metar or taf involving vertical measurements are given in height, AGL in other words.
Met offices are getting smarter by the days, they calculate weather phenomena very accurate these days, given a basic wx situation (wind and velocity) they calculate the CBs position and moving direction, they calculate the base and soon you have the forecast as you see here for yourself.
Looking at the date it may fit actually into the seasonal wx condition, something serious which came up the Atlantic, not unusual.
Watch CNN or other wx broadcaster, their mathematical models are very precise about these occurrences.