The 80% tax means that 80% of the allowance would be subject to tax at the standanrd rate of income tax which is 20%. Not 80% tax charged on the allowance, the actual tax charged if the 80% figure is correct would equate to 16% as detailed in the next paragraph.
For instance say for a £30 meal allowance, 20% (£6) would be tax free and 20% tax would be charged on the remaining £24 = £4.80 tax on £30. which brings the tax down to about 16% tax on the entire £30 allowance for a meal taken or not. Of course meal allowances are all different depending where the allowance is paid and the rate of exchange on the day etc.
The only way to escape any tax on meal allowances would be for the airline to arrange for meals to be taken in the hotels on long haul flights paid for by the airline. then no income tax could be charged. That is impossible on short haul flights unless a night stop is involved.
This is nothing new, the allowance system has been taxed but not on 80% of the allowance. To state 80% tax will be charged on an allowance is very misleading and not correct.
Most airlines gave up this old fashioned type of meal allowance payment system decades ago and have an hourly rate instead that is less complicated to work out.
As the allowances have already been subject to income tax HMRC would not be able to go back 6 years to reclaim tax as someone mentioned because they reached agreements for the amount of tax in the previous years that has already been paid.
The worst that could happen is if 100% of the meal allowance became taxable like other income is.
In the past BA cabin crew have enjoyed high incidental payments with a lower basic salary. The take home amount was the all imprtant point, that was fine then. None of the incidental payments are pensionable though.