HMRC will process the deduction if you file it via either a structured email or a tax return. Why? Because we operate a ‘pay and file’ system whereby the authorities first and foremost trust the taxpayer to get it right. The claim would, however, fail should HMRC ever open an investigation.

The cost of an upgrade would not meet the ‘necessary’ test where the cost of the lower class ticket was borne by the employer unless there was a medical reason why the employee could not travel in the lower class cabin. The ‘necessary’ test applies to the additional spending in question and not the requirement to travel.

An analogous case would be where an employer provided an employee with perfectly serviceable stationary. If the employee decides instead to write only on gold edged paper from Smythson (and pays for this personally) is the cost of that paper a“necessary” expense of employment for that individual? Absolutely not.

(note that this applies only to a payrolled employee who does not receive a flat rate travel package)

source: am a tax accountant.