It is reported by Sky News. As specified, this is the lowest level of threat since 2014.

Home Secretary Priti Patel says the UK national terrorism threat level has been changed from severe to substantial which is its lowest level since August 2014

- Sky News Breaking (@SkyNewsBreak) November 4, 2019

The British terrorist threat system has five levels: low, moderate, substantial, serious and critical.

The level is set by the Joint Center for the Analysis of Terrorism and the Security Service (MI5).

In May, British intelligence agencies said they had prevented 19 terrorist attacks in the country over the past two years.