The rules are specific to each FF plan, so if by spending BA miles you mean redeeming an award from BAEC then you would be best advised to ask in the BA forum. There are awards called "Oneworld" awards offered by AA/BA/QF/CX (and presumably also the other 4 OW airlines) but each has significant differences in rules. As a guideline in the AA plan it would cost 220K miles for J with travel up to 50K miles allowed (the mileage is summed from stopover to stopover). Rules at
http://www.aa.com/apps/AAdvantage/Vi...artnersContent
I'm sure BAEC has similar info on their web site but it is harder to find!