Virgin have essentially stolen from me

SW17

The flight you changed to, which was subsequently cancelled, must have been quite a lot more expensive than the first one (APD alone would be approx £100 less in economy than in PE). Have you looked into cancelling for a full refund (you have this right) and rebooking? Alternatively, is there another flight which suits your needs more than your original flight, which you can ask to to move to (also your right)?