If James I and VI had been killed, and the House of Lords destroyed, then the average Briton would have been expecting Spain to swoop like a shark to complete the slaughter.
That simply would never have happened.
Philip III of Spain would have kept his distance for the same reason as Henry IV of France and Navarre and every other European crowned head. They could not openly support the murder of an anointed monarch by his own subjects, especially not commoners.
That would be setting a dangerous precedent, that would not have gone unnoticed by the Spanish, French and Navarre people. Better to preserve the mystique of monarchy than claim another country for Rome.
We know this because both Philip and Henry refused involvement in many earlier plots by disgruntled English Catholics. Philip, at least, had also been approached by two members of the Gunpowder Plot, before it had been put into operation. He once again declined support.
In the upper echelons of Parliament and court, there was a pervasive suspicion that Catesby, Fawkes et al., had been working on the express orders of the Vatican or Spain.
It was never proved and probably wasn't true in either case. Guy Fawkes was questioned about the involvement of the latter, while under torture. He eventually spoke about the entire Gunpowder Plot and named names. He also confessed to details which had been cooked up in Lord Salisbury's imagination.
Yet Fawkes always denied the involvement of Spain. He had no reason to hold back at that point, so it's probably correct.
There would be plenty of precepts upon which the Catholic monarchs in Europe could invade, not least the inevitable lynching or arrest of their own ambassadors or papal agents. But the cons would always outweigh the pros, while the divine office of kingship was at stake.
But moreover, what would be the point? Jacobean England frankly wasn't that important to start with. Now it was a politically crippled, practically bankrupt nation, with a highly hostile population, that would be difficult to defend and hold, when the Protestant countries in the north rushed in to take it back.
So what's in it for Philip then?
Why not just fight the Dutch in Flanders, as normal, without the hassle of an occupation complicating matters? The Protestant armies would be one nation down, as England would have too many troubles at home to participate right now or at any time in the foreseeable future.
For continental Europe, there were simply bigger fish to fry, as the Thirty Years' War would soon demonstrate.
But there's one last, huge consideration. Catholic Ireland would be unlikely to be so docile. Shock and sympathy immediately after the explosion would soon give way, if anti-Papist feeling erupted so violently across mainland Britain. It would turn to antagonism and fury instead.
If there was ever a possibility of a Catholic Uprising, as a direct consequence of the Gunpowder Plot, it would have been Gaelic. Supported by the Scottish Highlands, Spain, France and maybe the Papacy, the Irish might even have won.