Counter terror police say they have obtained a warrant to hold a 28-year-old suspect for up to seven days.
The legislation cleared the House of Commons in Sir Keir Starmer's final days as prime minister.
Major incidents were declared in North Wales and in Glossop, Derbyshire in the Peak District on Sunday.
The couple's daughter tells the court her father is a "jealous, conniving, narcissistic villain".
France's President Emmanuel Macron praised Starmer's "decency" and contribution to Ukraine and the security of Europe.