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HMRC internal manual

Employment Income Manual

The general rule for employees expenses: relief is limited to expenses actually incurred: example

An employee has to travel on business. Her employer pays her a travel allowance that is enough to buy an airline ticket. She actually buys a rail ticket at less cost. She can deduct the expenditure she actually incurred, the cost of the rail ticket. She is taxable on the payment from the employer less the amount she actually spent.