If the flight has left the airways system in the UK there is nothing to be gained from cancelling a flight plan or even cancelling IFR...........simply make one's way to destination.
Actually - there is.
Scenario. Two or three IFR aircraft inbound to a non-radar airport outside CAS, estimating the airport within a few minutes of each other. As IFR separation still needs to be applied then no matter what the conditions the first aircraft will get no delay for a procedural approach, the subsequent ones will get EAT's issued and sent to the hold with the possibility of a wait, which could be up to twenty minutes depending on how many ahead and what the approach interval is. (this is not hypothetical - it does happen).
Cancel IFR and the above does not apply of course.