EIM31632 | The general rule for employees expenses: relief is limited to expenses actually incurred: example
From HM Revenue & Customs · Employment Income Manual
An employee receives from their employer a payment to meet the cost of hotel accommodation when they have to stay away overnight on business. However, instead of staying in a hotel, they sleeps in their car. They are taxable on the amount they receive from their employer and are not entitled to a deduction for overnight accommodation because they have not incurred any expense.