None of the LA's I have worked for will accept an orangery as exempt nor have any where I submit my guvvy work.Its a grey area. I agree a totally solid roof on a conservatory makes it subject to B regs in theory, but clearly the LA aren't interested as there are now thousands of companies offering solid roof conversions.
It's academic anyway since it's unlikely a conservatory structure could be made compliant....it certainly wouldn't be worth the aggravation.
LAs will accept orangeries, even with small lanterns to be exempt. So you don't need a full clear roof at all.
As for the Co's advertising they do this work, some have LABC type approval others submit an app for each individual scheme, however there are others who claim it is exempt this included a co based in the LA area where I used to work, we were just waiting for a solicitors enquiry or neighbour complaint for one of these schemes, I also know that another LA were taken them to task in the Midlands as one of their BCO's had contacted me to see what I knew about the Co.