Brexit rules will apply. I would recommend checking the rules section for your league. Generally though they either have to be a consistent international (unlikely for a youth player) or you will have to meet several criteria. Transfer fee is just one of them. The wages you pay to the player often needs to be higher also (average for a first team player for example). The reputation of the league they come from also matters.
Check the rules section and think whether or not the player can meet the criteria.