Hopefully a non-contentious topic (unlike my last post on solar heating!)
Next year (according to the grand plan) we should be replacing our decrepit conservatory. Now that the regulations on conservatories seem to have changed (no stipulations about percentage glass etc), it would be possible to re-build with solid walls and roof. ( Of course the possible PD changes may even make sizing a little more flexible but that's no big deal from this POV).
Clearly, this enables decent insulation values to be achieved (which is desirable in any case) and so the structure starts to come close to being an extension, from a building regulations viewpoint, especially as footings and floor slab will be re-built as part of the process. Of course, there are other elements to complying with BR but I guess my point is that the distinction between the two sort of merges.
Other aspects that come to mind are that heating in a conservatory has to be controlled independently of the main house system (but it seems that would be implemented in any modern extension), while wiring in a conservatory should probably be 'external' (but that sort of blurs with RCDs on everything anyway). Also, the doors into the space have to be 'external', but they are in place anyway and I probably wouldn't change them in a hurry. (Too big to get easily as internal doors.)
I suspect that keeping it as a conservatory would allow for a more 'creative' approach in design (as not every last BR box needs to be ticked) but then again creativity isn't always as good as boring old good quality building. (How do those glass boxes they build on the telly get past building regulations anyway? Maybe it's special magic glass.) And of course, it would be a bit cheaper for the same build if there was no BCO involvement.
In the end, the real question is what are the pros and cons of having either a conservatory or an extension, cost and hassle during construction notwithstanding? Is there an essential practical difference (house value, danger of higher council tax etc etc) or is it simply a philosophical distinction at that point?