The taxes are separate from the total fare and MUST be returned. The admin fee is the £480, so it's £730 - taxes (refunded) - £480. If the total fare for the later trip is £800/£600 whatever is left can then be used to rebook. If higher you pay more, if lower its less.
If you don't turn up you just lose the money.... No recourse,
So contact the airline, give them the reasons and see what they do. I once had a problem whilst staying in Philippines and due to fly back with Qatar. I explained the situation, and even though I should have taken an admin fee hit and then pay the difference but due to the circumstances they waived all charges for me.
If if you don't try, you don't get.