From the BSAC Position Technical Diving paragraph F specs in the first link you posted, "BSAC Tech does not mandate the use of manifoled twins with isolator".
Really. Rather they promote independent doubles. Which is not that different from sidemounting except the first stages are behind you. Which is IMO less safe than in SM. Headscratcher for sure.
Does not mandate means you get a choice. You can use other configurations. They do not mandate indie twins either. I don't think that means they 'promote' them. You need to look at the actual courses
ADP can be done in wild and crazy configs everyone here would condemn. Actually everyone does it in with manifolded twins or indie twins plus a stage.
Accelerated Decompression Procedures - British Sub-Aqua Club
Twin Set Diver - British Sub-Aqua Club
Technical SDCs - British Sub-Aqua Club