If I'm not mistaken, using BA miles would also mean having to split the ticket so lose the accompanying protections. If you combine two partners on a ticket, the metrics change again (the award becomes mileage based) and I think it's usually even worse value...
http://www.britishairways.com/travel...e/public/en_gb
I'm not sure but I suspect it's distance flown, rather than the distance from A to B..