Five minutes as far too long for a safety video - it will be ok the first time but imagine watching it for the sixth time! Will you be paying attention? Personally I prefer the old fashioned manual safety brief - gets all the info over in a minute, but them I'm old fashioned, and they are rarely needed now that we have small screens every third row in an airbus.
An interesting question not just related to the BA safety briefing but generally is how many passengers don't speak english? Is that a problem? Could you have videos in different languages where you have seat back videos?