You did not make clear if you were in fact a diamond, but even if you were there is no promise of an upgrade to the Club Floor at international properties:
Lounge access is not available at Hyatt Resorts worldwide and hotels outside the U.S. and Canada.
As I see it, it was your decision to use the hotel phone to call the US Customer Service center. Did you try the UK reservation center (th e number is on the back of the Diamond Card and then ask them to transfer you if they could not help?
My advice with regard to your first complaint is (1) read the GP T&C and (2) move on.
With regard to your second complaint re foreign exchange charges, I would have started with discussing the issue with my credit card company since the amounts involved were (or should have been) equaled out. If because of an incorrect charge you lost money because of currency fluctuations then the hotel should reimburse you.