If there was a flight plan and this was prior to the "responsible person" then ATS were seriously at fault for missing the fact that a flight had not arrived.
I don't think there is ATS at Colonsay (or whichever unmanned island strip he was en route to). That's kind of the point. No one to close the plan with .. but also no one to take action on non arrival either. The pilot was not in contact with any ATC unit at the time of the crash.
The consensus seems to be that many UK ATC units simply can't be bothered with flight safety mesages and UK pilots have no problem with that.
How many countries have ATC who are proud to announce that they ignore Flight Safety Messages?
I really suggest you have a read of this GA Safety Leaflet. It tells it the way it is (for VFR flights). Namely that if you file a flight plan then your arrival airfield and alternates will get it (no mention of a requirement for transit airfields to have it too, let alone do anything with them). It also explains in detail who is responsible for Alerting action and how this works depending on whether your departure and arrival airfields have ATS or not. As it's written by the CAA I expect they know the way the UK system works. And one which everyone (except a solitary individual) seems to accept and live with.
Paragraphs 3,4,5 & 7 are the relevant ones (it even mentions mountainous terrain flights .. where once again, surprise, surprise, the Alerting action is placed on your arrival airfield or a parent ATSU (having been told about your non arrival by a responsible person).
The CAA also put their contact details on this leaflet. So if you can't accept the way they think it works (which has been iterated here by many other people), then get in touch and put them right as well
In the meantime ... units in the UK will deal with 'transit' Flight Plans as they always have.
CAA GA Safety Sense Leaflet 20A VFR Flight PLans