Pilot, your company may have negotiated a fixed amount for uniform maintenance allowance a bit like the military did. Be aware these are usually minimums.
In the case of RAF officers I calculated that the tax relief at basic rate over 5 years would pay for the complete replacement of your basic uniform through stores based on expected life of each item. This allowance made not consideration for replacement of any No 1 or No 5 items. These were effectively extraordinary expenditure and their costs, supported by receipts if necessary, could be claimed for in-year. Naturally this would only give you the tax relief element.
In Mrs PNs case her allowance negotiated by her membership body was derisory. I calculated her laundry costs and additional replacement items. These were duly claimed (several times more than the basic allowance) and accepted without question by HMIR.
Your union dues are admissable.
Your travel expenditure (T&S) where this is more than the actual expenditure can be itemised and deducted. If your company pays you less than 45p/m you can claim the difference.
Gift Aid is often misunderstood. If you give to charity they get the tax back at 22% (I think) but as a higher rate tax payer you also get 18% back.
Maybe employ a tax consultant for a year of so, or get TaxCalc and read through their notes.