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.
I agree. Before the DS TS PS BS, the obligations were not always very clear. In particular, while an approach controller had certain responsibilities as regards separation of IFR flights, the responsibilities of the pilot were unclear.
There is still no
legal obligation to follow the instructions/clearance of a controller in class G (outside an ATZ), but the responsibilities of both parties are now set out in CAP 774 for all types of service.
BS: Unless the pilot has entered into an agreement with a controller to maintain a specific level or level band, a pilot may change level without advising the controller/FISO.
TS: Pilots may select their own operating levels or may be provided with level allocations by the controller for the positioning and/or sequencing of traffic or for navigational assistance. If a level is unacceptable to the pilot he shall advise the controller immediately. Unless safety is likely to be compromised, a pilot shall not change level or level band without first advising and obtaining a response from the controller, as the aircraft may be co-ordinated against other airspace users without recourse to the pilot.
...
In order to reduce RT loading and increase flexibility, pilots who require to frequently change level whilst receiving a Traffic Service should request a 'block' altitude to operate within.
DS: Controllers will normally provide level allocations for positioning, sequencing, navigational assistance, or to achieve deconfliction minima. If a level is unacceptable to the pilot, he shall advise the controller immediately. Unless safety is likely to be compromised, a pilot shall not change level without first obtaining approval from the controller, as an aircraft’s flight profile may be co-ordinated against other airspace users without recourse to the pilot.
PS: Controllers will normally provide level allocations for positioning, sequencing, navigational assistance, or to achieve deconfliction minima. If a level is unacceptable, the pilot shall advise the controller immediately. Unless safety is likely to be compromised, a pilot shall not change level without first obtaining approval from the controller, as an aircraft’s flight profile may be co-ordinated against other airspace users without recourse to the pilot.
(If that makes me sound like a fan of the DS TS PS BS, I'm not -- but it does clarify that aspect nicely.)