The person in this role is 'acting' as the role...But their rank is usually a grade less.

A quartermaster sergeant in the British Army and Royal Marines is usually a non-commissioned officer or warrant officer who is responsible for supplies or stores. However, in the Army this definition is extended to almost any warrant officer class 2 who does not hold a sergeant major appointment, as well as a number of staff sergeant and colour sergeant appointments. In the British Army, quartermaster sergeants are frequently addressed and referred to as "Q".

Examples of quartermaster sergeant appointments include:

Battery quartermaster sergeant
Company quartermaster sergeant
Quartermaster sergeant instructor
Regimental quartermaster sergeant
Squadron quartermaster sergeant
Artificer quartermaster sergeant