Unless the EU and EEA countries agreed to do the same then it would just divert most people to traveling via Dublin, Paris and Amsterdam with split ticketing.
Well flights taken from those places don't count towards UK emissions, and thus it becomes someone elses problem. The rest of Europe is eventually going to have to resort to similar charging strategies - or concede subsidies to aviation of tens of billions of euros per year.
Also going anywhere in Europe via Dublin would be utterly insane. You can't reach it quickly with surface transport and its hundreds of kilometres in the wrong direction to go anywhere in Europe!
Paris CdeG would be the obvious option.
Until the economics of greener flying are viable flying will continue to generate a lot of fossil fuels.
Unless electricity prices drop to near zero, which they might (but I am not holding my breath), they will never be "viable".
The modern aviation industry is based on an unlimited supply of incredibly cheap kerosene, that is going to go away one way or the other.
There are ways that the aviation industry could cut kerosene use, but it won't do them whilst kerosene remains ultra-cheap.
Abolition of business/first class, scrapping most of the windows in the plane, fuel stops/breaking up very long distance flights etc.
People shouting climate emergency isn't going to change the electoral reality that flying is seen as essential by many. I fly much more than average but I live in a flat, I don't drive and I don't eat meat.
Aviation emissions are already on order of 10% of UK emissions, this fraction is going to rise precipitously on the grounds of continuing aviation demand growth and cratering emissions everywhere else.
Once electric cars go (with electricity decarbonisation rapidly progressing) they will be a substantial portion of all remaining emissions.
My carbon footprint balances out at around the national average. Not flying would seperate me from family, I care more about them than carbon emissions. There are many millions of Brits and British residents in that situation.
And there are many tens of millions more who are not. Voters of this worldview will also tend to be inefficiently distributed, being concentrated in a handful of urban seats.
If a government is faced between stringent energy use cuts in the domestic environment (or imposing veganism on the population) or clobbering frequent fliers, they will clobber frequent fliers every day of the week.
And I (and the politicians) don't really want to deal with the nightmare of a 3-4C world, so the emission cuts will have to happen.