Hi Tom,
First, just a thought: you might get more people looking at your post and replying if you re-word your subject to focus on the particular legal issue of the inquiry. (E.g.: "Does US patent pending preclude UK filing?" or "Does failed PCT filing open the UK market when US patent pending?") In that way, people with some knowledge of the issue will better spot it while perusing the general list and be more likely to jump in if they think they can help. (My philosophy/poli-sci background, e.g., predisposes me to read "Is this a free market" as a discussion of economics.

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I'm not a patent attorney, but did practice US patent litigation, so I'll take a stab at a general response which I hope helps a bit:
My understanding is that in most if not all countries, all it takes to preclude you from getting your patent (on grounds of it's not being new or "novel") is for someone else anywhere in the world to have made public the same invention -- whether patented or not -- prior to your first publication of the work. (In the US you have a year from the time you first make your invention public in which to file for a patent, but I believe that is not the case in Europe.) If the patent examiner finds "prior art" which "reads on" the proposed claims of your patent, you're out of luck. The prior art does not have to be patented, it only has to have been made public.
The advantage to someone having a patent is that they can preclude you from making, using, selling, etc. the invention if their patent grant extends to cover the jurisdiction you are operating in. If the prior art is not patented, then it just keeps you from getting a patent yourself. (A US patent would keep you from making/using/selling/importing the invention in or into the US.)
So my guess is that you're generally right as to A and C (although the lawyer in me wants to caution you that there could be extraneous facts not provided which might alter that). B would be right as to the conclusion, but I wouldn't call it having "a 'worldwide exclusive' on the idea." As for D, I don't think you have that quite right, but it involves more complexities of the various countries' patent prosecution practices than I can go into.
Hope this helps (and I invite any patent attorneys here to step in and correct whatever I've got wrong or add any other helpful info re the specific questions raised).