The IMCr holder has always been able to use their privileges fully in all Class D airspace within the UK as they were in the previous Special Rules Airspace.
Not so. I disticntly recall when there used to be a list in Pooley's of those Class D CTRs in which one was not permitted to use the UK IMCR in IMC. One such was Cardiff; one week a colleague went down and had to scrape around under the scud to get in; a week later he could have taken radar to ILS in IMC when the rules changed!
Did extending the IMCR to all Class D cause problems? No. An important point and one I made at the recent CAA/EASA Press Brief - the UK IMCR privileges have been developed since 1970-ish as experience has been gained. And of course, as air transport movements into regional airports has expanded hugely.