Not the point! You HAVE TO DO what the MOT requires, daft or not, or face the consequences sooner or later. If the structure is non-standard your insurance company HAS TO be told or your policy becomes invalid in law as you have not disclosed a material fact.
The roof panel is secondary structure as although nothing will fail immediately without it, it has a significant effect on the torsional stiffness of the shell. That makes it unarguably a part of the monocoque.
Somerford list the roof at £407.11 including VAT so on a Mini worth what they are now, a new roof may add much more value than its cost. I would suggest fitting a new one, or finding a good used one on a scrap car, Then it truly is a fully legal plug welding job in place of the original spots, or hire a spot welder for a day.
Edit: The reason I take that view is that the average person, including me, is going to spend an enormous amount of time trying to make a welded repair invisible, with no guarantee of success. I would hope that a professional would also go that route to minimise the time charged to the customer. A professional panel beater or amateur with good skills in that area might go for the welded job on their own car to save money, if they have the time, knowing that they have the skill.
Edited by tiger99, 25 March 2017 - 01:02 PM.