PEGI is a voluntary system - the game developers use the PEGI guidelines to rate their own game, then use the PEGI logo on the game case to display the PEGI rating. Because of that I always thought it was a recommended rating, rather than a legal one. A BBFC certificate will only be a requirement if the BBFC deem it so. I imagine that in the case of Reservoir Dogs, the game was rated as 18 by the BBFC, which means it would be illegal to sell it to anyone under 18, and so if they didn't put the BBFC logo on the box they'd be breaking the law. Perhaps with Bioshock the BBFC haven't deemed it worthy of an 18 certificate and it's only the developer's recommendation?
As an example, I remember back in the days of the Playstation when the ELSPA system was used - it had tick boxes showing what age groups each game was suitable for. I was surprised to see Command and Conquer had a big 'X' next to all the age groups except '18+'. Yet it wasn't rated as an 18 by the BBFC. When I tried to buy Tenchu, on the other hand, it had the same ELSPA rating (only had a tick next to '18+'), yet it also had an 18 cert from the BBFC, so I was stopped from buying it because I couldn't provide ID.
So is it the same thing with Bioshock? I do see your point though - if it's supposed to have a BBFC cert on the box and they've somehow omitted it, then it would have to be recalled.