It all depends on alot of things. One thing seldome actually creates a complex thing, most often it's a number of things which works together to create a situation. Such as the German defeat at Stalingrad.

For example Boudicca could maybe have driven the Romans out of Britain. What happend then is very hard to know.The Romans could've decided that it wasn't worth it, they could get really pissed-off and come back and hit ten times as hard, decide to invade again when things have colded down, make use of client kings rather than direct conquest or other solutions. Decisions being based on many other things than Boudicca herself. The price of different enteriprises, list of priorities for the empire and such. It's very hard to generally tell what MIGHT have happened.

As to fate, I do belive that there may be a big general fate. Such that the world will one day end (as is the case when the sun finally dies) but I don't think that every minor detail is predestined. Although I don't really see fate makes a free will no existent. I think that fate and free will could well exist at the same time, but that it would mean that free will was submitted to fate. But then again what is "fate" and what is "free will" is a rather philospical question.