Just to let people know, last week I had the promised letter about the 35kg roof rating from the dealership where I got my MG 5. As Les' correspondence suggests, they said there's a bit of paper about it to stick in the manual. I told them I would pick it up next time the car went in for a service.
I wasn't actually expecting to be able to put anything on the roof when we bought the car, so to me, this was a nice bonus. Though I did laugh out loud when the sales guy told us the roof rails were purely decorative when looking at the car -- "wait, what, you're actually serious?!"