You seem to be working on the assumption that you knew or strongly suspected that he would actually choose "split" despite apparently pre-committing to "steal". If you believed him to any significant extent it is clearly better to choose split and hope he keeps his word (unless for some reason you think this makes you look bad enough that you'd rather willingly give up an expected large fraction of 6000 pounds instead).
I found this to be a very interesting method of dealing with a modified Prisoner's Dilemma. In this situation, if both players cooperate they split a cash prize, but if one defects he gets the entire prize. The difference from the normal prisoner's dilemma is that if both defect, neither gets anything, so a player gains nothing by defecting if he knows his opponent will defect; he merely has the option to hurt him out of spite. Watch and see how one player deals with this.
http://www.youtube.com/watch?v=S0qjK3TWZE8