It looks very much like MI5 and the police in the U.K. have interrupted a plot to conduct a massive terrorist attack on Easter. 12 suspects have been arrested in a hastily re-planned police operation (after the now-former senior Anti-Terrorism Officer in the U.K. committed a blunder that nearly revealed the investigation).
The search is now on for the bomb-making "factory", and likely for other suspects implicated by the arrests and confiscations of materials.
These arrests also are rapidly turning into an international incident as 11(?) of the suspects are Pakistani Nationals, with 10 of them having entered the U.K. on student visas approved (in part) by Pakistan. Substantial ties to al-Qaeda and the Pakistani Taliban are reported to have been found as well.
This is an on-going story, and likely further information will become public shortly.