I'd do 'in a flash'. I associate 'in a jiffy' with something more practical, such as 'I'll be there in a jiffy' or 'I'll have it for you in a jiffy', rather than just indicating speed of movement.
I don't think Bertie would actually say 'dream rabbit'. But having said that, I have nothing else to suggest.
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I don't think Bertie would actually say 'dream rabbit'. But having said that, I have nothing else to suggest.