Have you rotated and checked the wheels for run-out (wobble) while on a jack?
Do the same with it in 1st gear with the engine running - taking suitable safety precautions of course.
When you take it down for a wheel balance & alignment,m be sure to let them know and ask them to look.
I have, and they do seem to be spinning slightly off center, as are the drive shafts and diff pot joints. How much run out is acceptable for a mini? Or should there be absolutely none at all?
I really doubt it would be the Pot Joints or Drive Shafts, however I think what you've found with the Wheels themselves should be further looked in to as with big wide wheels, it wouldn't need much to be able to feel and notice an issue.
Bear in mind, this could be the Wheels of course, but also could be the way they mount to the Drive Flange or the Drive Flange itself. Check the Wheel Nuts are in fact doing up tight on the Wheel and not bottoming out on the Stud or the Drive Flange.