INTM333710 | Double Taxation applications and claims: arrears of tax: Introduction
From HM Revenue & Customs · International Manual
An ‘arrear of tax’ is any amount of tax which remains unpaid by the taxpayer but is due and collectable.
If relief is due to a non-resident applicant/claimant who has arrears of UK tax, see INTM333720 before you make any repayment of tax or authorise relief at source.