Hi,
It talks about 'As far as practicable, check that all airbags fitted as original equipment are present and not obviously defective.' in the MOT inspection manual.
As your car is re-registered as not a Z3 and something else then there is no 'original equipment' as it is a kit car, you just so happened to use some Z3 parts as part of your 'kit car' build, right.....
Also I have the passenger airbag present and fully functional, with no warning light on the dash. However the SRS airbag embossed text is covered with a well placed, rather nice looking, badge.
Mine went through the MOT and they were more concerned that people might trip on my exhausts as they extend out a couple of inches from the car! so I got an advisory
Good luck!