B is at fault from an inter unit coordination viewpoint. However the CAA is more at fault from an airspace and regulatory point of view.
Regardless of the situation, unit A agreed to accept traffic from adjacent unit B at FL55.
In the normal situation it is the responsibility of unit B to ensure that the aircraft complies with the coordination agreed before it leaves their area of responsibility.
In my view it is an open and shut case on an inter unit basis. However the CAA is largely at fault and I believe that this example highlights failings in the regulator.
This situation is inherently unsafe in design exactly because there is no controlled airspace.
The controller at B can not clear an aircraft any further than the boundary of controlled airspace i.e. "abc123 cleared to leave controlled airspace on track AB climbing to FL55" or ABC123 cleared to leave controlled airspace to the west climbing to FL55"
Overall, the controller at B has no control over the flight after the airspace boundary.
One would have to investigate the inter-unit agreements and also the demed separations in force between the two holding areas and inbound/outbound routes as well as the RMAs.
For example should the controller at B put the aircraft back into his/her hold until it was level at FL55 or at least kept it within their controlled airspace? If so where is that written down.
If the border between the two unit's area of responsibility is a distance from the controlled airspace boundary the the following clearance would both comply with the CAAs requirement only to clear aircraft to the boundary of controlled airspace and clearly tell the pilot that they are expected to be at FL55 for the next unit.
"ABC123 cleared to leave controlled airspace to the North climbing to FL55. Unit A will accept you level at FL55".
Unfortunately, that CAA approved clearance can result in this situation we are talking about.
If however the inter unit boundary is the edge of controlled airspace then it is easier to define and the clearance is less open to dangerous errors;
"ABC123 leave controlled airspace to the north climb FL55 level by the zone boundary."
Of course if the airfields are very close then the holding area from airfield A may enter the controlled airspace for airfield B
Then of course we have the old problem of the pilot havign left controlled airspace deciding to do some general handling before the next holding exercise. Being outside controlled airspace, that pilot can turn off the radio, play round from the MSA to FL whatever and on completion level the aircraft at FL55 and tune in airfield A and report inbound to the beacon at FL55.
Not to mention that joker G-WXYZ could decide to do a few holds at airfield As beacon at FL50! in IMC with no radio and no transponder at all.
Overall, everyone must agree that whatever the cause the effect is a dangerous situation. It is incumbent on the CAA as the regulator to act to prevent dangerous situations. I believe that simple Class E airspace round such airfields would improve the safety situation drastically without impinging on any VFR pilot's right to go as they please in VMC.
Regards,
DFC