TCTM04901A | Sums disregarded in the calculation of income if certain conditions are satisfied
From HM Revenue & Customs · Tax Credits Technical Manual
Payments of expenses received by voluntary workers
The Tax Credits (Definition and Calculation of Income) Regulations 2002, Reg 19, Table 7, Item 1
Any payment in respect of any expenses incurred by a claimant who is engaged by a charitable or voluntary organisation or is a volunteer, but only if the claimant receives no remuneration or profit from the engagement, nor is treated as possessing any employment income in respect of that engagement.