Tomkins,
I agree with you. However that is not nor was ever the purpose of the differing taxation levels applied to 'expenses' to be used abroad.
The idea of taxing the overseas supplementary pay at a different rate was that it was considered that local taxes would be paid abroad thus the idea of 'double' taxing was brought up and the lower tax rate applied in the UK.
At no point was the supplementary pay ever meant to be retained and used to bolster basic pay in the eyes of HMRC. If that became the case then those expenses would be taxed at standard UK rates.
So, the HMRC then applied a taxation 'test' to a number of Airlines, BA included. The idea being to clarify the spending levels of crews down route in order to assess the taxation rate to be applied.
Oddly enough, as most crew look at their 'additionals' as just that they failed the test and subsequently a new tax level will be introduced.
Once again this entire exercise was quite poorly communicated by BASSA apart from BASSA issuing a list of what expenses were to be spent downroute.