There are two issues here. FREA and subsistence allowance (SA). If you get SA, each company will negotiate with HMRC (UK) what proportion of that hourly rate will be taxed at your marginal rate and what will be tax free. To gauge that actual expenditure matches the tax free element of SA, pilots are sometimes asked to keep receipts of their actual expenditure over a period to check that they are actually spending the allowance on subsistence. Any SA paid over what is used is regarded as income and taxed as such. Obviously, this is averaged out over the workforce and not applied individually. If you feel that your expenditure exceeds your allowance, you may present receipts to HMRC.