It was about Spain mate. The only way WIlberforce was able to get the aristocracy on side was because they'd farmed the crap out of their plantations in Jamaica etc to the point where the yield on the land had fallen away, hence they no longer had use for all the slaves. This meant the output from the non-battered soil in Spanish owned territories would swamp the sugar market and give the Spanish a massive advantage. Funny that suddenly lots of people had a change in heart and decided slavery was bad. There were a few who did it for 'the right reasons', but it was about power and domination, as per usual.
Again, there are 2 distinct points here:
1. The significantly more important point, Britain (and by that I mean it's Citizens and by extension it's government) were the ones who first pushed and and succeeded in ending of practice of slave trading. This point is paramount and undisputed.
2. As to your point regarding why it was stopped: this is of significantly less importance. And doesn't warrant as much musing as we've already dedicated to it. And I'm about to continue to in the following paragraphs. But in respect to your exchange, I will.
Again, your claims as to why (i.e. It was about Spain) is still wrong. The overwhelming evidence that we have about this period, from the setup of the Antislavery Quakers, the Abolitionists Anglicans, the Testonites, the writings of Clarkson, Ramsey, Pitt, and Middleton. The establishment of Slave Abolishment Societies, the Campaign of the Sons of Africa, the Books of Equiano and Cugoano, the Clapham Sect e.t.c., the establishment of similar Societies in Spain, Italy, Portugal and France. The lobbying of Government etc...
We know way too much about the period and the original writing of those who were present and their reasoning that it would take an ignoring of the know evidence to conclude that a want and will to end the suffering of humans in bondage wasn't the underlying principle of the movement.
But to engage your international point, in so far as there were any significant reaction to external forces that helped, it wasn't Spain but France. As Britain was engaged in hostilities with France. And soon there after the Abolitionists ( Including Wilberforce) But more specifically Stephen concocted a plan to put forth a bill to stop British Citizens from participating in supplying slave to foreign colonies Britain was at war with.
In essence making participating in Slave trade and act of treachery against the Kingdom. The economic gains of Spain, France or any other Nation was, if existent at all, of minute importance.
So, no. An opposition to the horrors of human bondage was by far the flagship reason for ending slave trade.