Bliar and his cronies dismantled the 'Foreign Earnings Deduction' back in 97, that allowed you to remain resident in UK but if out of the country for more than 9 months your tax rate was 0% - this type of deduction is still available in a majority of EU countries but not so here...this was certainly the case for civilians and one assumes transposed over to the armed forces
As such the only alternative available is a negotiated/agreed 'tax allowance' to cover sundry incidental expenses whilst you are away - this has been used across the board by many companies not limited to aviation - you cannot achieve as high a saving as under 'FE deduction' but at least it is something - as example I have agreed with the IR an allowance of £100 for every day I am outside the UK on company business (this is not paid but is deducted from my tax liability so if I am away 100 days a year I get £10,000 deducted from my gross taxable income) - whilst a friend of mine who works for a UK based multinational gets £200 a day which his company has agreed with the IR!
The attitude of the tax inspector that I dealt with was very positive - he saw the logic that if I was sweating my backside off in Nigeria as example I should have some 'benefit' over and above working in the UK and with the canx. of FE Deduction this was the obvious way to do legitimately do it within existing IR procedures.