EIM00640 | Employment income: earnings from employment: customary payments in respect of services
From HM Revenue & Customs · Employment Income Manual
Even if a lump sum payment is not made under a legal obligation, it is taxable as earnings within Section 62 ITEPA 2003 if it is customary within a particular employment for such payments to be made to a particular class of employee.
The case of Corbett v Duff (23TC763)(see example EIM00645) illustrates this principle.