Thanks for that, PPrune Radar.
These were plans with no errors or things which the UK NAS system didn't like.
For plans which requires some manual intervention in the UK NAS system, they join a queue, so their turnaround time will depend on how many other plans are ahead in the queue for correction.
So are there plans that are accepted by IFPS (and therefore compliant with airway structure and RAD) but still unacceptable to the UK NAS system and therefore requiring manual intervention? What sort of problems might there be with the FPL that require manual intervention at that point?