That would be if they were acting as a middle man. Before player power a club would contact an agent for help getting rid of a player. They are doing services for the club.
But these days in football it's more common for a player to have sole representation, with the player paying for his agents services, usually % of the wage. Madness that they even ask for a fee from the buying clubs, even more so that the club's pay it.
For tax purposes HMRC soft on it and let them get away with it on a dual services basis.
https://www.saffery.com/news-and-events/press-releases/2021/new-hmrc-guidance-football-and-payments-to-agents-april-2021