Blair.
The best thing about the current government domestically is the relative strength of the economy (i.e. no big swings of boom-and-bust), which is due to the disciplined policies of Chancellor Brown. If we had a different Chancellor, whoever that is would muck it up and undo Brown's good work. Therefore, Brown must stay as Chancellor, and therefore Blair has to stay as PM rather than Brown. There are, of course, numerous reasons for not wanting either of them in either position.