Flybe do EDI/NQY during the summer months, loads were reasonably good last year but I don't think there is the demand for two carriers on the same route.
EDI to BOH or PLY (is that the right code for Plymouth) would be good though and would be more £40 than £20, Flybe's cheapest EDI/NQY fares are £34.99 by the looks of the website.