Not all that long ago, the ANO used to include restrictions on certain Class D airspace inside which mandatory compliance with IFR by IMC rating holders was not permitted.
And this is where I believe the "...only outside controlled airspace" limitations for foreign IR holders came from.
In the past the IMC rating did not allow IFR "in controlled airspace" but airspace which is now class D was not defined as "controlled", with "controlled" referring to Class A only.
When airspace was re-classified, and JAR came about, the IMC rating was redefined to include "controlled airspace, class D and lower" however the definition for foreign IR holders was omitted, probably by accident. The reason I reckon it was a clerical error was because the CAA used to have on their website that "foreign IR holders can exercise the same privileges as an IMC rating holder". However because of an omission (i believe) in the ANO the "outside CAS" limitation was never updated for foreign IR holders, and as "Class D and below" is now referred to CAS, and not just class A, then this effectively banned them from any airports in class D airspace (IAW IFR).
I haven't looked for a while, but I believe an IMCr attached to a CAA licence could also still have this limitation somewhere in the wording.