Not sure about the UK, but here in Canada the direction changed about a year ago for situations like this.
Previous to that change in direction, the AR was to "statue" when that ball goes into the goal, and then give the referee the information that players were in an offside position. The AR's angle here is impossible to tell whether those players interfere with or impact the defender on the line. All the AR knows is that they're CLEARLY in an offside position. The referee's job becomes "did they interfere with or impact the opponent's ability to play that ball?" If the referee deemed that the players in offside position interfered, then the flag goes up, whistle goes, IFK.
The change of direction (Aug 2017 I think it was?) was that the AR should now raise the flag and "own the offside". In most cases, the referee can tell who is in an offside position and can make the determination immediately as to whether there was actually an offence, and make the call immediately.
The reasoning for the change of direction is pretty much what @one just identified... credibility.
Previous to that change in direction, the AR was to "statue" when that ball goes into the goal, and then give the referee the information that players were in an offside position. The AR's angle here is impossible to tell whether those players interfere with or impact the defender on the line. All the AR knows is that they're CLEARLY in an offside position. The referee's job becomes "did they interfere with or impact the opponent's ability to play that ball?" If the referee deemed that the players in offside position interfered, then the flag goes up, whistle goes, IFK.
The change of direction (Aug 2017 I think it was?) was that the AR should now raise the flag and "own the offside". In most cases, the referee can tell who is in an offside position and can make the determination immediately as to whether there was actually an offence, and make the call immediately.
The reasoning for the change of direction is pretty much what @one just identified... credibility.