Seeing a Britain without Roman influence would be interesting but I have to agree with twovultures that the most interesting migration PODs aren't on this list. What about butterflying the Mongol conquests or the Germanic migrations?
i voted Norman invasion the list since i find a persisting Anglo-Saxon England to be very interesting. going away from the poll, the Spanish invasion and conquest of the Aztec Empire posits even more interesting possibilities
I voted the Anglo-Saxons because I find a surviving Celtic nation very interesting since otl screwed them hard, though no Roman invasion may do the same.
Not on the poll is the Seljuk invasion since that had far reaching effects on the Middle East, Europe, and arguably world history since it gave a boost to Islam at a time it was very weak.