Hello,
I have 2010 Mini hardtop. I recently had this issue couple of weeks back, and is now fixed. I just want to post this incase if any one come across this issue.

Issue:
Mini is juddering on acceleration (jerky motion) around 40-50mph. It seems okay at other speeds, or at least not noticeable.

Fix:
I took it to mini dealership, and immediately they were able to suspect that its ‘spark plugs’ issue. They have replaced the spark plugs and all looks good. It costed my around $400.