According to PlanetGTA:
The BBFC have been given seven hours and forty-three minutes of cutscenes so they could properly review the game.
The five hours and twenty minutes of gameplay probably contained short clips of every questionable thing in the game, and is in no way representative of the total gameplay time, obviously. Also, there is no guarantee that all of the cutscenes were given to the board, but my guess is that they were. If you fail a mission in the game, you get different dialogue in the cutscenes the second time and maybe even more times, so several versions of each cutscene may have been sent to the BBFC.