I looked for clarification of this question here
Single seat cover on motorcycle - Bike Testing - Public Category (Ask questions about your car and the MOT) - MOT Testing Forums
the experts say that they may record the number of seats visible but the only MOT requirement is security of rider's seat and both sets of footpegs. It is then legal to ride with a pillion as long as there is adequate seating.
I'm sure there is a 7 seat Zafira discussion somewhere in that forum as well. I would have thought that the MOT tester will pull out foldaway seats. The problem would be with the versions where 6 & 7 sit in your garage until you need to slot them in.
Sq