Just for your thoughts really.

I am having an extension built at home. I am the client and my builder is the "Principal Contractor". This is all theoretical as no mention of the CDM regs has every been mentioned but bare with me.
I drew plans and submitted the planning permission, I supplied the builder with all relevant information in regard to the build of the "out houses" that being knocked down and where all of the services were.
Building control were contacted at the correct time and visited site. Once the building was taken down, building control visited again.
On this visit he was unhappy with the neighbouring building's (semi detached) 15 year old extension as it was tied into our old out house. This meant that we had to redesign the new building and install additional lintels (basically to prop up next door). All of this resulted in a change in build order etc.

Does this mean that the building control officer has responsibility as Principal Designer now?

The old ACoP made it very clear that Building Control, Historic Scotland, English Heritage, etc. were not to be considered as "participating in design". I haven't checked the new 2015 Guidance.

Either way, the Building Control Officer cannot be the Principal Designer as he is not in control of the design phase of the Project.

The way I read it, the BCO was telling you the design was insufficient.
The designer had not taken into account your duties to your neighbour

I am pretty sure statutory functions like Building Control are protected against being considered a designer as long as they act within their powers as set out in the relevant act. it is a similar situation with planners.