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There's going to be little solid evidence of it as elections are so few and far between and very rarely occur outside term time.
The last 10, stretching back to 1979 have all been in April, May or June. Before that it was October 1974 and that would have been in term time.
The last one that was in December was in 1923.
(Which is meaningless given the voting age was only lowered from 21 to 18 in 1969.)
Students are much less likely to vote when at home too.