Not sure about per diems exactly but the UK Inland Revenue are trying to stop any tax free element of duty pay at airlines as the current deals run out. BMI Baby, Flybe and others are facing problems in the near future, I believe.
A rise in per diems might give the IR the chance to alter things and it will always be in their favour.
It maybe an idea to push for a better pension, as this is tax free\deductable at present.