The general rule for employees: expenses: dealing with expenses: how to resolve a dispute
A request for a deduction is not a claim to a relief. So a dispute about a deduction cannot be resolved by the machinery for making and determining claims in sections 42 and 43 TMA 1970.
The mechanism for permitting relief for an expense is to deduct it from earnings to arrive at the amount of an assessment or self-assessment. So, the machinery for resolving disputes lies in an assessment and an appeal against that assessment, see EIM31706. Many disputes can be resolved without the use of formal machinery, see EIM31710.