To clear up a misconception,
YOU DO NOT GET £850.
The figure is an additional amount added to your "tax -free" earnings allowance.
If, for example, you were earning £850 a year over the higher-rate threshold, you'd pay 40% ? tax on it. by claiming the fixed allowance, you'd get £340 back (or that much more in your year's take-home pay.
Many trades and professions have a negotiated agreement with the taxman.
AFAIK, in the absence of any employer contribution (your per diem allowance IS a contribution) you are entitled to claim the fixed allowance irrespective of what you ACTUALLY spent.
If you spent more than the fixed allowance, you have to weigh up if it's worth the effort of making up accounts and submitting a tax-return to claw back the overpaid tax.
Usually, the taxman's test is that the expense was"necessarily and wholly" incurred in the performance of your job. In the case of the notebook, if you can prove it's used solely for work, it's claimable, if there's a private-use element, the allowable expenditure will be reduced proportionately. likewise the headset (your Union could put up a strong case for the inadequacy of the standard airline issue h/set, though the "elf n' safety " aspect of inferior hearing protection, may mean the employer would strenuously deny their offering was lacking.
REMEMBER, you do NOT "get back" what you paid out.....you only get it allowed as tax-free income.....but the plus-side is that it "Top-slices" your tax,- that is, the allowance is at your highest rate.
I'm NOT an accountant nor a tax-expert and i'm rapidly losing touch with the arcane rules of tax and employment...so don't take this as gospel!