From :

Geeks WorldWide, which is a reliable source for movie production info, is reporting that The Flash will film at Warner Bros. Studios in Leavesden, UK. This studio will be no stranger to The Flash star Ezra Miller. He’s filmed Justice League there, as well as two Fantastic Beasts movies. The studio was also home to Wonder Woman and is currently being used for Wonder Woman 1984.

Read the Full Story