Yes agreed. But there are vast swathes of UK airspace where it is not obvious which is the relevant frequency because areas overlap or are not well defined.
With respect to MATZ I have to disagree. The frequencies are listed in the AIP (and other up to date in flight guides etc) and also on the half mill map. In the case of CMATZ you call the controlling authority - an example is Scampton where one would call Waddington.
Noting correct frequencies should be part of flight planning.