Will do nothing but damage the economy if brought in unilaterally.
International flights will just ensure they can fuel at the other end in every possible scenario thus depriving airports of a revenue stream. Seeing as the UK has so many airports this could easily lead to bankruptcies and closures of smaller international airports. You could also see situations where a plane would have to take on fuel in the UK - TATL or similar - doing a hop arrangement like BA001 does - it can't get off the ground with enough fuel to cross the Atlantic anyway so it drops in to Shannon to fuel up and let the passengers use US preclearance.
Domestic flights would just get dearer, increasing the cost of doing business in the UK and reducing their viability, see above for what that could do to airports.