As
@es1 requested:
There are two points why you should not apply advantage (technically, you don't play advantage) from an offside.
1. As the offence occurred?
2. How can it be an advantage, when the keeper has the ball in his hands and the potential offence was 20 yards up the FOP?
The best outcome is that the goalkeeper has procession, which is not advantage, it is just "play on".
If the keeper kicked the ball directly into touch, would you call play back and award the free-kick? Technically, you should as they have not benefited from the advantage, so you should award the original offence!!!!
If the keeper has the ball after an offside, please shout "seen it, keeper lets keep going" - not offside. As an observer at Supply League level, I would see this as you not understanding the advantage principle and its creates a development point in your report - all all the implications that brings under the new scheme.