There are many different venues to check out a movie in and around Leicester Square.
The Prince Charles Cinema on Leicester Place is a great value, selling tickets around £10. While they do offer new releases, PCC offers many showings of older movies, often with themes/accompanying events like Pixar movie showings, Quote-Alongs, Sing-Alongs, the entire Die Hard Trilogy, etc.
The Vue and Odeen cinemas offer new release movies with student ticket prices from £13 to £15, with upgrades naturally being more costly.