Medical letters saying it is not appropriate for claimant to attend WCA

I though I knew this but there was something on the ATOS website that made me doubt myself. 

I have a client who has been asked to go to a face to face WCA. Its clearly not appropriate or safe for them to do this and I am going to ask the GP for letter saying so. I thought this would have to go to ATOS at their Bristol address but when I looked on their website it said that the decision about whether a claimant had to attend an assessement or not was made by JC+/DWP which indicates the GP letter should go there- anyone know which is correct?

The decision whether to call someone for an assessment in connection with the WCA is in the ESA regulations and therefore falls to be made by the Secretary of State.

But where that assessment is conducted is at ATOS’ discretion, if you’re punting for a home visit send it to your local Medical Services office, ours is Birmingham