Yes, this character is John Uskglass.
I've already answered this part on another SE site, and most of the same arguments apply here.
Who else could he possibly be? A magician of great power, enough to revive the dead without fairy assistance, and who changed the writing upon Vinculus's body. Vinculus was the personification of the Raven King's book; the writing upon him was the Raven King's prophecy - who but the Raven King could change it to show something new and different?
The man wore black boots and a black travelling coat. He was half-stooping, half-kneeling on the snowy ground beside Vinculus. For a brief moment Childermass thought it was Strange –but this man was not quite so tall and was somewhat slighter in figure. His dark clothes were clearly expensive and looked fashionable. Yet his straight, dark hair was longer than any fashionable gentleman would have worn it; it gave him something of the look of a Methodist preacher or a Romantic poet. "I know him," thought Childermass. "He is a magician. I know him well. Why can I not think what his name is?"
Out loud he said, "The body is mine, sir! Leave it be!"
The man looked up. "Yours, John Childermass?" he said with a mildly ironic air, "I thought it was mine."
It was a curious thing but despite his clothes and his air of cool authority, his speech sounded uncouth –even to Childermass's ears. His accent was northern –of that there was no doubt –but Childermass did not recognize it. It might have been Northumbrian, but it was tinged with something else -the speech of the cold countries that lie over the North Sea and –which seemed more extraordinary still –there was more than a hint of French in his pronunciation.
[...]
"Does the land hereabouts belong to you, sir?" [Childermass] asked.
"Yes."
"And where is your horse? Where is your carriage? Where are your servants?"
"I have no horse, John Childermass. I have no carriage. And only one of my servants is here."
"Where?"
Without troubling to look up, the man raised his arm and pointed a thin, pale finger.
Childermass looked behind him in confusion. There was no one there. Just the wind blowing across the snowy tussocks. What did he mean? Was it the wind or the snow? He had heard of mediaeval magicians who claimed these and other natural forces as servants. Then comprehension dawned on him. "What? No, sir, you are mistaken! I am not your servant!"
"You boasted of it, not three days ago," said the man.
-- Jonathan Strange & Mr Norrell, Chapter 67 (emphasis mine)
A mysterious figure with great magical power and a strange accent, who claims to own both Vinculus's body and the surrounding land, and whom Childermass has boasted of serving - come on, there's only one man this can be.
Besides, Strange and Norrell confirm using their magic that John Uskglass is in Yorkshire at that very moment, but later departs and becomes once more unlocatable:
-
Norrell did the magic, naming John Uskglass as the person they sought. He divided the surface of the water into quarters with lines of glittering light. He gave each quarter a name: Heaven, Hell, Earth and Faerie. Instantly a speck of bluish light shone in the quarter that represented Earth.
[...]
In a tone of wonder, Norrell said, "I think we may have succeeded after all! It says he is here. In Yorkshire!"
-- Jonathan Strange & Mr Norrell, Chapter 66
-
In the silver dish of water the speck of light flickered and disappeared.
[...] "He is gone," he said. [...] "It is very odd," continued Norrell, in a tone of wonder. "What do you suppose he was doing in Yorkshire?"
-- Jonathan Strange & Mr Norrell, Chapter 68
His appearance is extremely significant for the ending of the novel.
Up until this point, it has seemed as though Norrell and Strange have been the ones pulling the strings of English magic. Their different approaches to the theory and practice of magic, and the growing strife between them (admittedly fuelled by the gruesome twosome Lascelles and Drawlight) has been the focus of the novel. Like Childermass, we the readers believe that they hold all the power and the destiny of English magic in their hands, and that their choices will determine its future.
Up until this point, too, the Raven King has been spoken of only as a distant, shadowy, semi-legendary figure. Their attitudes towards him make one of the biggest differences in philosophy between Strange and Norrell, but we the readers have been left unsure what to think of him. His existence in the past seems likely, his continuing power in the present day doubtful; he has apparently never manifested himself or his power over the course of the book.
NOW, suddenly, up he pops, seemingly out of nowhere. We might assume he appeared because he was summoned by Strange and Norrell, but he doesn't go to them, and instead goes about his own business. Casually he performs the feat of raising a man from the dead, which was so far beyond Norrell's abilities as to perpetrate the entire fiasco with Thistledown; equally casually, he rewrites the entire prophecy that inspired Strange's involvement in the world of magic. It's humbling to see how helpless Norrell and Strange are in comparison to him. This is the true power of English magic, and we'd never really glimpsed it before.
Just to add even more strength to the blow, we then hear from Vinculus that Strange and Norrell were never really in control of their own destinies. In the big picture, they were just the Raven King's pawns:
"So?" said Childermass, stung. "That is not so very trifling, is it? Norrell is a clever man - and Strange another. They have their faults, as other men do, but their achievements are still remarkable. Make no mistake; I am John Uskglass's man. Or would be, if he were here. But you must admit that the restoration of English magic is their work, not his."
"Their work!" scoffed Vinculus. "Theirs? Do you still not understand? They are the spell John Uskglass is doing. That is all they have ever been. And he is doing it now!"
-- Jonathan Strange & Mr Norrell, Chapter 67 (emphasis mine)
Admittedly Vinculus is hardly a reliable narrator, but on matters of prophecy, he does tend to know what he's talking about. He's already spoken many truths which nobody recognised or appreciated at the time. His very body is attuned to the Raven King's intentions and prophecies; I'd say he's worth listening to at the very least.
It's the Raven King who is pulling the strings, and has been from the very beginning. It was he, through his prophecy and his agent Vinculus, who brought Strange and Norrell together in the first place. He showed up in England for his own purposes, to resuscitate Vinculus and rewrite the prophecy, not because of the summons.
This reveal is part of what makes the ending of the book so stunning and unexpected. Another part, of course, is the disappearance of the two main characters to another dimension, leaving others to carry on their work. In some ways it's an unsatisfying ending - we've identified with Strange and Norrell all this time, and wanted to see a resolution to their conflict and a glorious future for English magic (not to mention Strange's marriage!) - but there's no doubt it's an audacious move by the author, and one that leaves us (or at least me) almost wanting to read the whole book again from the beginning.
