As I understand it, if you can prove your "centre of life" has moved outside the UK and spend no more than 90 days per year (over midnight) averaged over 3 years in the UK you pay only a small amount of income tax. Basically, the work you do within the UK, ie. standby's, groundschool, flight time within the UK, is taxable. it's your choice whether or not you pay NI but all of our fellas do (VS).
There is no tax payable in France unless you spend more than 90 days there either.
The revenue used to grant instant exemption but, due to abuse of the system, now want proof for at least 12 months of moving abroad.
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What The Hell's It Doing Now?