Only if they can prove at the time that you could have purchased a comparable flight at the time for £200 (and not a cheap one via Brussels and then Amsterdam before a third stop over in Paris). Probably hard to prove, and you would like to think that you would not have booked a £400 flight when a £200 one was available. If we were talking the difference between 1st and 2nd class then they would have a point, but I assume you are not here.
It is a bit like buying materials for a CE more than at market rates. They could assess it at what would have been open market rates at the time.
It is a bit like buying materials for a CE more than at market rates. They could assess it at what would have been open market rates at the time.