Bookie
That is interesting - my T-shirt came about after changing level under a basic service while IFR in IMC - well in the days before it was called a traffic service. I suggested that I had no obligation to inform of a change of level and was told firmly to the contrary - in fact I even raised the issue on here at the time. I seem to recall the view was the controller was correct. I recall the controller very pleasantly pointing out that if I had reported FL45 IFR then their assumption was I would remain at that level and traffic that may be higher or lower would not be passed (albeit I fully appreciate that they have no obligation to pass any traffic). There was no argument that I was fully entitled to change level just that I should notify them of the change first.
With hindsight personally I think it is pretty silly to change level under IFR (especially in IMC) without notifying the controller under a basic service but am interested that you feel there is no "legal" obligation to inform the controller.