LONDON (Reuters) - The threat level facing Britain remains at critical, interior minister Amber Rudd said on Saturday, as she praised the police for making a  very significant  arrest following a London train bombing which injured 30 people a day earlier.  Police arrested an 18-year-old man earlier on Saturday following the attack in west London.   The threat level remains at critical and there will be further investigations and part of the operation later today,  Rudd told reporters.    This is a very significant arrest. The police have made very good progress.  She added that it was too soon to know whether the culprits behind the attack were known to the authorities. 
UK threat level remains at critical: minister