The above response is absolutely rght. Gettysburg, had the South won, could well have been like the Revolutionary War's Battle of Saratoga, which convinced the French to enter the war on the American side.
The British were reluctant to back the South, given the fact that they had already abolished slavery and certainly did not want to aid its continuance in the US. However, a divided America offered some benefits for the British Empire, and with a southern win at Gettysburg, they may well have entered on the confederate side anyway.
In addition, it is important to note that the losses at Gettysburg, in particular on day three with Pickett's Charge, gutted the heart of Lee's Army of Northern Virginia (particularly in the officer ranks), and effectively ended their offensive potential. For the remainder of the war, they stayed in southern territory and remained a defensive force.