Just googled WFTA this is in their rules
Rifle accessories allowed
A single rifle sling - no additional straps are allowed.
A sling used to carry the rifle or as a means of steadying the aim. Such a sling shall be attached to the rifle at a minimum of one and maximum of two points when a shot is taken. The sling may be unclipped from the rifle when not in use.
So why is it an issue in the UK?
Daystate mk3. Valdada IOR 9-36 x40 .