Firstly, I would not go to anywhere that isn't recommended by others or that specifically deal with Wheel Alignment....Protyre are one such company that are well versed (and trained) in 4 Wheel Laser Alignment.
The bushes are difficult to diagnose when only marginally worn but if the alignment shows it to be out on the rear suspension and you haven't slammed into any kerbs then chances are the bushes are worn.
Finally, I would not imagine that BMW would entertain bushes under warranty as they would be classed as 'wear and tear' but worth a try I suppose
Cheers, Trevor