Could it also be that "mainstream" news are actually trying to verify information and/or obtain confirmation from other sources? All of that is done in an attempt to avoid promoting false information. People tweeting do not do any of that

Could be. Of course, the UK press can also be subject to “D-Notices” that prevent publication of certain sensitive stories. In cases where a story may be subject to a D-Notice, your only hope for information is to go to alternative or foreign media.