Mary I's surviving son and the Anglo-Spanish succession

What if Mary I had a son with Philip II who survived them and became both Prince of Wales and a Habsburg heir?

His upbringing would decide where the crisis fell: an English-formed heir could face Habsburg opposition, while a Spanish-formed heir could provoke rebellion in England and Ireland.

The heir's upbringing is the decisive variable because the two crowns could not simply be joined as institutions. An English upbringing could produce a comparatively tolerant Catholic monarch whose habits provoke resistance among harder-line Habsburg subjects. A Spanish upbringing would prepare him for more direct royal authority and could trigger rebellion when he tried to govern England by those expectations. The comparison with James VI and I illustrates why a monarch's original court and power base shape how successfully he can move into a different constitutional system. Ireland adds another fault line because replacing established local intermediaries with trusted Spanish appointees would create fresh opposition. Either route is likely to produce a major political or religious schism somewhere within the composite monarchy.