At the risk of being accused of Eurocentrism: Rome abandoned Britain in 410 AD. Centralized government and administration disappeared quickly, and local warlords gradually emerged as the power centers. At the same time, Britain became the target of repeated incursions from Picts, Danes, etc.
The British Isles had all the agricultural, mineral and metal resources required to sustain a civilization. The Irish monasteries had copies of all of the foundational documents of what would become western civilization. Ireland also very quickly developed a form of Catholicism and a set of laws that were far more egalitarian than was normally found in that era.
The POD is the rise in the fifth century, post 410, of a leader who unites all of Britain south of Hadrian's Wall, re-establishing centralized government and defending the island from invaders. (Or co-opting them into settling peacefully.) Over the next two centuries, Ireland and Scotland join Britain in some form of federation, voluntary or otherwise. The British Isles develop a civilization independent of the mainland, which is going through Gibbons' decline and fall. You could even butterfly in the earlier development of moveable type if you like, perhaps by some monk who wants to escape the scriptorium.
With that head start, and with a form of Catholicism that lacks the stultifying effects of Galileo's era, it's possible Greater Britain could have been the seedbed for a much earlier industrial revolution and all that followed. Space travel by 1700 then becomes far more likely.
