If the UK government were to unilaterally place duty / tax on jet fuel, and much dependent upon the level of taxation, the airlines will simply tanker fuel in from countries where the fuel is cheaper and / or not taxed.
Nb. Many airlines already tanker fuel as a viable method of reducing the uplift required from airports where fuel is expensive - indeed at my airline we do a "should we tanker ?" analysis before every flight.
That said, tankering fuel increases the weight of the aircraft and this in turn causes more fuel to be consumed, however the additional burn required to lift the tanked fuel is more than offset by the fuel price differential.
Accordingly, and somewhat ironically, the overall effect of a tax on aviation fuel is likely to cause more fuel to be consumed - which is probably not quite what the 'Greens' intend any such a tax achieve.