Going nicely from what I read of your blog.
If asked to turn right at roundabout as you describe and choosing wrong lane then that would depend on what you are now telling other road users and where the road ahead goes to…
If left lane and indicating right then that is a no, no
If not indicating and the road ahead is not restricted (or a dead end !!) and you take that correctly, then nothing can be marked against you - just makes the examiner think about a route deviation to get you back on course 
There is not a place on the marking sheet for ‘trainee took wrong safe & legal route’ - you could just say that you thought the examiner said ‘road ahead’ 