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).
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).