In the movie, clips of a Mosquito dropping 'Highball' bombs were used, as the actual film footage, with a Lancaster dropping 'Upkeep', was still Classified, and remained so until the early 1970's.
The earlier sequences, showing the initial tests, as shown to "Arthur Harris" when "Barnes Wallis" gets to see him, show a Wellington, and is actual footage. although I believe some editing was done to partially disguise the shape of the weapon.