The truth is, it's a mixture of tax grab and enviro-posturing. Nothing more, nothing less. Will the extra tax stop people flying away on holiday? No. Will the tax revenues raised be ringfenced and used for direct environmental protection or research and development of better technologies for environmental protection? No. So the exchequer will keep (and subsequently waste) the money and the politicians can proclaim themselves eco-warriors.
One thing I rarely hear mentioned is that plants metabolise carbon dioxide. If you plant enough trees, you should be able to fly as many aircraft as you like, as long the carbon balances. Surely someone must have done the maths on this............