For me, there's some resonance in the names of the two service providers: Distress and Diversion are there to provide an alerting service and deal with aircraft in distress and diverting; FIS is there to provide a FIS. The units are equipped, and those staffing them are trained, for these purposes. Forgive me if this is stating the obvious.
In terms of coverage, don't forget that the flight crew of the majority of UK-operated airliners, and many others, monitor 121.5 on a spare box. This, providing you don't mind your distress or urgency call perhaps being relayed, provides effective coverage down to ground level.