Is it possible that another programme was planned to be shown but was dropped (perhaps because it featured someone such as the late Caroline Flack?)
No, that particular episode of Celebrity Mastermind was in the schedules all along.
There's absolutely nothing unusual about this programme at all, there are gaps in the schedule every week before they get filled eventually. There was one last week, there were a couple of others this week (including, as mentioned, another the same night). Usually nobody notices because they aren't in such a prime slot and Eurovision fans and Whovians aren't combing the schedules for clues.