The Dash 8 was dedicated to the PLH-NQY-LGW operation; the route had to carry the full fixed costs of the aircraft and crew; and it was a double-drop operation with PLH, which meant that the DOCs per roundtrip were high due to four landings per roundtrip. I suspect all of this means that the trip cost for a fully-costed Dash 8 is probably not dissimilar to the direct operating cost of a 737 LGW-NQY-LGW where the aircraft + crews are already paid for by other activities. It is not comparing like with like, or anything like it.
With the IOM + INV sale to Flybe, I think you will find that the slots will not be going to an "unfriendly" home who will use them to compete with BA.