You should have a look here:
http://www.flyertalk.com/forum/briti...os-2015-a.html
Based on your location, I assume you are starting in JFK and that the 100 000 miles are BA Avios ?
Using CX, you will have to transit in HKG. JFK-HKG is over 8000 miles and the cost for a one-way in C is 100 000 if booked before 28th April; 150 000 after that date.
In Y, you would have just enough Avios for a return journey as it is 50 000 each leg.
I don't think it is possible to push to Japan using Avios with CX.