INTM333290 | Double Taxation applications and claims: Vouchers: What to do if the claimant wants vouchers returned
From HM Revenue & Customs · International Manual
You cannot return paid vouchers on HMRC forms (for example R185/R189) in any circumstances.
You should not return paid vouchers unless you receive a written request from the claimant or their formally authorised agent.
If you do return any vouchers you should make sure that:
you keep a complete list of them with the claim
you stamp them with the date the repayment was made.