The mileage levy proposal feels like a rather absurdly intrusive item. I'm also not quite sure how they're planning to administer it (i.e. if I split my travel between airlines, who is tracking what?).
It also feels like what is likely to happen as a result of a bunch of this is increased shenanigans by higher-frequency travelers to duck the tax. It isn't like the UK is likely to have much luck tracking an AF or DL ticket from CDG to JFK post Brexit...so I feel like a small cottage market will develop on that front.
Also, banning FFPs in the UK [for example] would likely result in folks simply flipping their "loyalty alignment" to AA [for BA] or DL/AF [for VS], unless the government wants to take a serious stab at telling foreign airlines what to do.
NGL, the answer would probably be to just go back to straight regulated fares and force airlines to cease fighting it out on amenity cuts...but that would hose Austrian politicians holidaymakers going to Ibiza.