It's hard to define the biggest British movie, as it could be measured in many different ways. Box office success, critical acclaim, cultural impact are some of the way to measure a movie's success. Some British movies that have been considered very successful and popular include:
Harry Potter and the Deathly Hallows – Part 2 (2011)
Skyfall (2012)
Titanic (1997)
The Lion King (1994)
Star Wars: Episode I – The Phantom Menace (1999)
The Dark Knight (2008)
Jurassic Park (1993)
The Lord of the Rings: The Return of the King (2003)
The Chronicles of Narnia: The Lion, the Witch and the Wardrobe (2005)
The Lord of the Rings: The Fellowship of the Ring (2001)
This list is based on box office performance, but again there are many ways to measure success, depending on the criteria.