That of course is your choice. As I've made clear in many posts, the players belong to the teams not the NT/u21. However, training English players benefits your team by giving you higher merchandising, and in the case of NT representation, that can be a significant increase in merchandising, paying a large chunk of the NT player's salary.
That is why I feel its in team's interests to train NT players especially.
But again, the choice is yours.