I can’t pinpoint either one in James Bay’s song; there is no problem for “La Belle…” though; what we can hear first is “Je saurais t’aimer…j’en ai rêvé.” The pronoun “en” in that case stands for “aimer”; this is the most likely antecedent (she has fantasised about loving him, not about knowing how to do it); you have to remember that that pronoun does not have a simple antecedent in many cases; it takes a lot of habit to detect justly that antecedent every time and that is also true for the indigenous speakers of the language; she had been dreaming or rather hoping that sometime she would be the prince’s lover and since she is so inclined to the relationship she thinks she can be assured of having enough love for the prince. The form occurs a little later again, but it’s changed: it’s “je saurais t’aimer … tu l’as rêvé.”; the prince is now taking back his place after having removed the dummy or usurper that La Belle took for him, and as she begins saying again “je saurais t’aimer … j’en ai rêvé.” the prince says the first part along with her (“je saurais t’aimer …”) but he is taking it up for himself, and as she is surprised because she now recognises she is not dancing with the fake prince but with the real one she stops her singing and in the place of “Jen ai rêvé” the prince goes on with the continuation of what he wanted to say and he says “tu l’as rêvé.”; this time the pronoun “l'” stands not for “aimer” but for “Je saurais t’aimer” and doing so he is justifying in her eyes that he too will know “how to love” her because she has fantasised, allegedly, that he would do so; all of that is not too logical but in the world of love and fables one tends not to bother too much with logic.

Independently of the occurences in artistic creations the difference to remember is one that has to do with the verb used. If the verb calls for a direct object (COD) “le” (l’) is used.

— Les oiseaux ont quitté leur nid…
— Je te l’avais bien dit, ils étaient déjà très grands.
(“l'” stands for un antecedent that you have to make up and things are rather awkward, messy if you try to be exact; but anyway it’s also like that in English; les oiseaux quitteront leur nid avant longtemps)

When the verb used is intransitive (COI) and that the preposition is “de”, then the pronoun to use is “en”.

• parler de _ Le prix des fournitures va monter; j’en ai parlé au chef de service.
• se servir de _ Donne moi le couteau! Je ne peux pas je m’en sert.
• sortir de _ Il était impliqué dans des affaires douteuses; il s’en est sorti avec un bon avocat.
• rêver de _ Une voiture de course…J’en ai rêvé!