Try and make a timeline in which England does not unify until the 19th century at around the same time that Germany and Italy do. For the sake of continuity, we will assume that the kingdom of Wessex, while not being able to unify England, still creates the Empire, and gets all of the colonies that Britain did OTL (is this ASB?). ITTL Wessex would be similar to Prussia IOTL. Is there any way for England to unify this late in history and still have world history remain similar to IOTL? If so, please write how in the comments.
Bolded yes; a Wessex with political/security concerns on its own island (Involving the interference of foreign powers at times; say, the French, Scots, or even the Welsh; provided they centralize as efficently as IRL, no one English kingdom would be capable of subduing them without opening their backs to rivals in the east) wouldn't have nearly the same level of wealth, naval forces, or will that would allow England's IRL colonization.
But you could very well see this with a stronger, longer Norse presence on the Isles. This could be accomplished by a greater focus on the British isles during the early Viking Age, with those who would end up conquering/setting further afield instead coming to the Danelaw for land and potentially future plunder in wars against the southern English. (Which may butterfly away places like Normandy, Norman Sicily, ect.) After all, if the land is fertile and close and the Jarl of whatever the Lodbrok dynasty decides to name its new kingdom, why bother taking the risk of sailing that far away? Say, throughout the 10th century you have raids/wars between the Danes and Wessexians that the former manage to win (Perhaps due to alliances with the Welsh who, seeing the collapse of their major threat Mercia under Norse pressure, decide its better to ally/co-operate with the new invaders against the Saxons, splitting Wessex's attention east during one of these wars. Hywel Dda would be a good candidate for this, seeing his ambitions and his ability to use the war against the English and his prestige as a tool to better unify his new Welsh kingdom and buy the loyalty of its notables with fiefs cut from Wessex). This could cut off Wessex proper from Cornwall to the extent the local Cornish could re-assert their recently lost independence, and the Danes could carve out Wessex's eastern dependents (Kent, a "Kingdom of London", ect.) as tribute-paying vassals. As Norsemen settle in northern England in greater numbers, you see enough of a demographic change (Or even a religious one; assuming the Norse don't convert quickly. Put them in sufficient enough concentrations and discredit Christianity enough by the collapse of Christian kingdoms and the burning down/looting of their centers of learning and proselytizing and its possible) that when the Danelaw starts to break down there isen't as much identification with the south as their used to be, and the minor Kingdoms have gotten used to their independence and soured at the idea of centralized rule. Maybe have the united Danelaw break down in the 1200-1300's, with smaller but still reasonably sized Norse-ruled kingdoms (York, Northumbria, ect.) takings its place among a collection of English states too weak individually to have one fully conquer any of the others. A Sengoku-type situation.
Granted, you MAY see something like
Brytenwalda re-emerge at some point, dredging up the old ideas of electoral monarchy. But even then it would probably resemble the title of Holy Roman Emperor more than anything else; a status of 'first among equals'. Then, as you start getting into the 1700-1800's, an intellectual movement towards Pan-AngloSaxonism starts to form as cultural knowledge spreads, the various dialects of English standardized somewhat due to mass printing (IE. English translations of the bible and wider-spreading literacy), and foreign pressure from the "Celtic" Scots, Welsh, French, ect. can create a sense of shared identity and the fall of dynastic prestige/importance of the nobility starts to break down.