IHTM14882 | Lifetime transfers: dating of dispositions: gifts by cheque
From HM Revenue & Customs · Inheritance Tax Manual
Gifts by cheque are not completed until the cheque itself is cleared since until that time payment under that instrument can be revoked.
It is important to bear in mind that
a gift by cheque is not complete until the cheque itself is cashed (following Re Owen, Owen v IRC [1949] 1 A11 ER 901 and for IHT in Curnock v CIR SpC 36[2003] STC (SCD) 283).
So be aware that
where transfers are made by way of several cheques, the simultaneous transfer provisions will operate only if the cheques are cleared on the same day. The date on which each cheque is drawn is immaterial. (IHTM14591) (or IHTA84/S43 (2) for settled property)