It is relevant, that is specifically the Home Nations agreement. If he gets British residency then he could technically play for Scotland, Wales or Northern Ireland as well - this agreement defines who can play for which home nation. It prevented Angel Rangel playing for Wales.
But he would be eligible for England provided the following applied. Not that it matters as he has clearly chosen Spain.
Taking on a new nationality
Article 7 contains the rules for a player to change nationality and thus represent a new national team. This criteria is identical to Article 6.1, apart from item (d) which requires a player to have “…lived continuously for at least five years after the age of 18 on the territory of the relevant association.”