IMC rated pilots, for example, are not allowed access to airways, or Class A airspace and for flights within Class D airspace, which includes the airspace around many of the UK’s airports, Minimum Descent Heights (MDHs) allowed for instrument approaches as well as minimum horizontal visibility distances are much more restricted than they are for IR qualified pilots.
You may like to read the above again, bose x.
There is no ban on "airways" (which just happen to be mostly in Class A in the UK) and there is no ban on Class D.
Perhaps some 1970s version of the IMCR did not allow access to Class D under IFR.
Interesting comment about unions. I know nothing about this one way or the other but I do know that any suggestion along these lines but within the ATC profession meets with a fierce response in these pages.