Ian W,
in that case, what would keep the EU from responding in kind? As I wrote in a previous post, for continental EU airlines, the other option would be to route around GB (or, should the centrifugal tendencies pick up in pace, around England only). For UK airlines, the option would be either to pay the likewise massively increased fees on much longer routes across the remaining EU or to select routes around Norway and Finland in the north or around Portugal, Spain and Italy in the south.
Now, would that have a bigger impact on European or on British aviation?