Airprox I understand that the Inland Revenue (IR) has given the PFA an Extra Statutory Concession (ESA) in this case. It was probably on the basis that the average PFA coach would incur expenses, that would nornally be tax allowable, in excess of the £20.00 charged. Such allowable expenses would include the costs of travel from the coach's home to/from the relevant airfield. For travel by car, the IR will usually accept up to 40p per mile, in these circumstances.
Self-employed examiners, who charge full flight test fees (say £100+ per test?) can also claim all tax allowable expenses, against such income, when submitting Tax Returns. Allowable expenses could include travel to/from airfield, CAA medicals, instructor renewal tests, instruments rating renewals, membership fees to AOPA, replacement headsets & other equipment, AERADS & other manuals & textbooks, and anything else directly related to that source of income.
So, on average, against a test fee of £100, an examiner may well have more than £20 of tax allowable expenses.