An early POD opens up things a lot, because it's before other people even have empires so really most places are up for grabs. However, you need to be careful about it. England achieved the remarkable balance of having strong state centralization early, and then decentralized checks on that centre later. This meant that you broke feudalism, but didn't lapse into absolutism. Having the Tudors followed by the Stuarts really set the stage for the right political system for an industrial revolution, which is what you need to have Britain truly dominant.
For these reasons, you need to be careful with some "successes". Success in the Hundred Years War means long military lines, meaning a larger state to pay for it, diverting resources away from private business as well as naval dominance. Conquest of the Aztecs and Incas is great for a windfall of gold, but most of it will go to the centralized state, meaning they can crush any pushes for democracy. It also encourages rent-seeking in the economy over entrepreneurship, limiting development. Spain suffered from this in OTL, and so do places like Nigeria today.
So if I had to have a large as possible British Empire, I wouldn't give them success in the Hundreds Year War. They want to divert north and subjugate Scotland instead, integrating identity-wise into a single British state. Without the threat of a continental power invading via Scotland, Britain is then safe to fully commit to a conquest of Ireland. More colonisation there before the religious divides could allow it to become an English-speaking British-identity place (at least in the cities and rural elites) before national religious divides complicate everything. If the Irish and English don't see themselves as different, there will be less subjugation of Ireland later on, and hopefully that island too can benefit from the British agricultural revolution. A more developed Ireland means more trade between the Isles, and even more of a naval headstart than Britain had in our timeline. You could also try to have Lollardy less persecuted, meaning its tenets spread among the working class in grassroots fashion across the British Isles. This primes the entire archipelago for the reformation (which we won't butterfly) and induces a Protestant-like savings culture at an earlier point, providing capital for trade missions. It could also serve to undermine the legitimacy of the Church and King, incentivising earlier movements towards representative governance.
During this time, British sailors increasingly sail to Iceland and beyond, due to the enhanced naval technology, following fish stocks, before discovering America in the late 1400s. Settlements follow in the next century, spreading down the coast, and logging in the area provides for cheaper ships for the expanding merchant fleet. The more adventurous explorers reach the Aztecs, and trade takes off massively, enriching the British middle class on both sides of the Atlantic. Meanwhile other European powers, mainly the Dutch and Portuguese, start founding trade posts all over Asia, as in OTL. The French, Spanish and British shortly follow. France quickly becomes the most powerful continental power (given no Spanish gold revenues), and has a series of wars with the British. The Brits, fearful France will catch up with them, have a series of wars in Europe. British privateers gradually push the French out of Asia, taking their colonies during the wars. Before long, British companies establish a dominant position in India and south est Asia by the late 1600s. New corporate arrangements such as stock exchanges and central banks are developed.
Meanwhile, in America, the Brits realise that the colonists, streteching from Newfoundland to the Mississippi delta, and increasingly in the Caribbean, are so numerous, they can't force them to be subjugated an devise a system of autonomy and federation. With triennial councils held in London to develop imperial policy. At first this is dominated by the united British Isles, but power is dispersed over time. The Aztecs become increasingly a protectorate of the British empire. France focuses on dominating in Europe, and eventually manages to gain control of Spain in a Franco-Spanish Catholic union against the now Protestant Brits. The Brits maintain alliances with the Dutch and Portuguese against them, taking Spanish colonies that have started in South America. British traders push further east, up to the Phillippines, China and Japan. The Brits found colonies in South Africa and East Africa as refueling posts.
The French population continues to grow and becomes an ever larger threat as it begins to throw its weight around. Eventually a Napoleonic Wars-analogue happens in the late 1700s, and Franco-Spain invades and annexes Portugal and the Netherlands, expanding its border to the Rhine. The Brits absorb their colonies in Asia and Africa, to keep them out of French hands. The industrial revolution begins 50 years early, and Britain surges ahead in technology. They increasingly fund German nationalist movements to try to form a united Germany, which eventually happens in the mid-1800s. By this point, Britain has settled most of North America, controls India and many of the East Indies, has protectorates over the Aztec and Inca empires, colonies down the South American Atlantic coast, South Africa, East Africa. Franco-Spain focuses on West and North Africa instead. China is being pressured by Russia from the North, and the British back China against them in exchange for trade concessions in cities, which, in fits and starts turns into informal empire and other powers locked out. Japan forms a Russian alliance and tries to modernise.
Eventually there is a great war in the early 20th century, with the British Empire and Germany against Franco-Spain, Japan and Russia. It's horrific for everyone, but eventually the former prevails. Japan is occupied, Franco-Spain is split up and loses its colonies to Britain. Russia loses its warm water ports in Asia and has to leave Mongolia and China. British supremacy is entirely confirmed. As oil picks up in the Middle East, the British establish protectorates there with British administration of the oil for the "greater good" of the world (i.e. British interest).
Is that big enough for you?