Well, no, it was actually a measure intended to make it more difficult for the British Government to declare war on the Union after the Trent affair, and to forestall international recognition of the Confederate Government.
Pursuit of the same recognition, by the way, that led to two Lee campaigns into the North, to seek a Confederate "Saratoga"-analogous victory that would bring recognition.
And to hinder the rebel war effort by negating the runaway slave laws. It was a combination of all three, and it was effective since what slim chance of rebel recognition remaining after Antietam died with it.
Pursuit of the same recognition, by the way, that led to two Lee campaigns into the North, to seek a Confederate "Saratoga"-analogous victory that would bring recognition.
Well one campaign at any rate. Had Lee's 1862 campaign been successful then British recognitition, or at least a push for a negotiated settlement, may have resulted. But by Lee's 1863 campaign all hopes of recognition were dead and most rational people knew it. The goals of Lee's 1863 campaign were to keep from having part of his army sent in a vain attempt to relieve Vicksburg and also to strip the North of food and supplies his army needed and which the inept administration of Davis was incapable of providing him.
The Trent affair? That had been successfully resolved eight months before Lincoln announced the proclamation. It's another of the southern wet dreams that Britain ever seriously considered declaring war on the United States. At most, the British might have considered recognizing the confederacy, but would have maintained their neutrality. A declaration of war would have almost inevitably caused a US invasion of Canada, which the British recognized they were in no way able to defend against even a small fraction of the military of the US possessed at that time.