This app gives an illustration of lower airspace - specifically it shows LARS (Lower Airspace Radar Service) units, and controlled airspace in lower airspace.
You can filter by day/time (for LARS) and by altitude (for controlled airspace).
I built this to help me visualise during flight planning which ATC units I might expect to talk to (or listen to) en-route.
The areas of coverage for 'Listening Squawks' is an approximation as there is no data set available defining their areas of coverage.
Data sources: ENR 1.6-22 UNITED KINGDOM AIP 31 Aug 2017 for LARS, and http://www.openaip.net/ for the rest.