Melchett, to a point. I did an exercise some years ago, easily repeatable, based on the whole life cycle cost of replao your working clothing over 5 years. The cost was exactly equal to the annual tax relief at source at 20% tax relief - for simplicity £400 relief = £80 and 5 year cost equalled £400.
If you replaced any #1 items, hat, gloves, shoes, uniform, shirts or #5 these were on top of maintaining your working dress. Keep receipts, claim extra.
The same applies to the difference between PTR mileage alloiwance and 45p/mile. Some people reckon it's not worth the bother but if you have to file a tax return, why not?