What exactly is a "good business case"?
Sticking a few ADS-B receivers round the coast probably only costs a few hundred thousand dollars and gives controllers a reasonably reliable view of where aircraft are. (Almost certainly better than they would get if they were relying on reports from the pilots, who will, after all, be making these reports using the same navigation avionic equipment that is feeding the transponder.) Transmissions from aircraft above FL290 can be received over 180nm away.
In areas where there is limited or no radar cover, the controllers can therefore reduce the separation minima and may be able to give more aircraft more optimal routes hence reducing fuel burn etc. HD should stop thinking airways!
There is little direct benefit to airspace users in the UK where most of our airspace is covered by 3 or more conventional radars, except that ADS-B technology should be significantly cheaper than sticking huge chunks of metal on big electric motors up large concrete posts and requires a lot less maintenance (and, therefore, outage).