According to accountsandlegal.co.uk:
"If you use your own car for business travel, you are entitled to claim certain costs as an
allowance against tax and that could reduce your tax bill" (emphasis added).
https://www.accountsandlegal.co.uk/tax-advice/uk-business-mi...In other words, something that "reduces [one's] tax bill" is a
tax deduction (or "tax allowance" if you will). A deduction is not the same as a reimbursement. A reimbursement would mean that the user is getting repaid directly by the government for his/her mileage expenses, which is not the case. The person gets a
deduction from the tax owed instead.
reimbursement: the action of repaying a person who has spent or lost money. "reimbursement of everyday medical costs" (Lexico.com)
reembolso: Devolução de importância devida: O cantor providenciou o reembolso do valor do ingresso do espetáculo cancelado. (Aulete Digital)
Baseado na informação prestada pelo Accounts Legal (UK), insisto que não se trata de um "reembolso" (sic) ou "devolução" (sic). É
dedução pura e simples. Que venham as evidências/referências em contrário!