Pull a _very_different_ Walker
Unlike Walker I would take my time, study the cultures of the Old World very carefully, and use my real and quite exstensive knowledge of Celtic and Indo-European studies to really get into the heads of either the Sun People, or, more likely, one of their Continental relatives. I already speak Irish, German, a little Latin, and a fair bit of Gaulish, so the Sun People tongue should have a number of very familiar features, making it possible to pick up relatively quickly. Indeed, the language seems to very closely resemble Old Common Celtic, and I have lots of material on that at home.
So, after picking up the local language, and learning the local power structure, I combine the role of bringer of irresitable techniques of war, with that of religious reformer. In this role, I don't set myself up as prophet, but use a local woman, preferably one with a reputation as a welitaa, or seeress, to express my ideas, and teach me more of the details of the culture. This is based on the role that such women played, even in the more patriarchal variations of the culture. They were regarded as mouthpieces or even incarnations of the Sovereignty Goddess, and so as essential to legitimate rule. The right welitaa could easily make me the power behind a native king, with much more fanatical support than Walker ever enjoyed. The religious reform is not intended to impose my own ideas on the religion, but instead to simplify it, regularize it, and impart memes that make the religion more able to resist Christianization and absorbtion by the Nantucketers. The struggle between Light and Darkness would probably be central to this, as well as a simple, conceptually unified pantheon, and the concept of Dedmaa, or sacred law.
Through the religion, I could introduce other useful technologies, techniques, and memes. The idea of Ulatis, or Sovereignty, as a guarantor of Dedmaa, for example, could be refined into the idea of a potentially universal state. I would also try to introduce an idea of Sun People linguistic nationalism - that all who speak the same tongue, share the same Dedmaa, and worship the same Gods are one Toutaa, that is to say, tribe, who must therefore have one Rix, one King, and one Ulatis, one State. This State would include an Assembly, and even the idea of limits on royal power. The government and ideology would be highly authoritarian, but not totalitarian.
Like Walker, I would need a secret police, but instead of using the, um, talents of a monster like Hong, I would be more likely to use a combination of my own people with the members of the priesthood who had come to share my vision and my reform. They would combine a power-political with an ideological role, and so be a lot closer to the historical NKVD or SS than to Hong's little torture chamber. Indeed, I have little use for torture and would discourage it, though probably not outlaw it inflexibly. Instead, simply "disappearing" people can work as well at quashing dissention, and is rather less brutal. There are some lively ways the notion of "night and fog" could be combined with the probable local folklore to give the secret police a mystique that would create as much awe and respect as fear.
(To be clear, I do not believe in or offer aplogetics for such methods today. But, the Bronze Age is a different time, and the building and modernizing of a nation a quite different task than maintaining a civil and democratic society in a country with long experience in self-government. I am not trying to imitate Hitler here, so much as the early-modern state-builders such as, say, Richeleu, or else modernizers like Ataturk.)
A number of ideas, ranging from the scientific method (sold as a mystical discipline), and modern medicine (ditto), to a modernizing army (sold as a warrior's society and discipline), an education system (sold as an indoctrination tool, probably expressed in local parlance as "glanosagion nertosagionc anatiyaas", or "purification and strengthening of the soul"), writing (sold as sacred runes), and mathematics (a great tool to keep the calendar and astrology in line), would be included in my package.
The best _place_ to do this would not be among the British Sun-People. This is too close to allies of Nantucket, and easily reached by sea. It would be wiser, if possible, to go quite a ways up the Rhine, near enough the modern Alsace and Saarland. This is an area rich in the kinds of resources that make industrial revolutions possible, with good farmland, plenty of space for expansion, and it's pretty hard for Nantucket to get too many boats or troops there. In any case, after a suitable interval, I would do my best to kiss and make up with the Island, so as to avoid attack and get higher tech by trade. This shouldn't be too hard, given that I have stuck around the Island a good bit longer than Walker, and that I threaten none of their vital or sentimental interests. In time, I would likely be viewed as a rather unpleasant ally, or as their version of Col. Kurtz.
Which would be their long-term mistake. The Island novels assume that over the long term the Nantucketers have a lock on modern medicine and methods, as well as control of a nascent world trading network, and a memetic superiority that outclasses any potential opponents. However, this new state will have more than the beginnings of modern medicine, a slowly developing scientific revolution, a Continental trade-network of its own, religious and nationalistic innoculation against assimilation by outside cultures, and, as it expands and absorbs one tribe after another (all poor, lost brothers who must have their national conscousness raised, of course), a superior base population. While the Islanders have a big head start, a unified Western Europe, centuries after my death, of course, would be a juggernaut very hard to beat.
Suddenly the Island will have competition, and the world may get to keep some of its richness and diversity as a result.