I work for an international maintenance company and I'm not (too) worried... we too, have a cunning plan, Mr Blackadder.
Hawkeye - I've read Airsound's points and they sort of agree with mine. A handover of major ATC control to another centre would possibly take weeks to say the least (but just installing capable RADAR might take years to develop). We can allow aircraft to overfly our borders, but aiding them further is a moot point.
If there is a real threat of a no deal - you will see planes leaving UK like flocks of Geese.