Doesn't help you Savage in terms of the amount you're discussing but I do have to do a great deal of money changing (cash) for work.
In the UK, bizarre as it seems, Marks and Sparks has a history of giving outstanding rates in big city centre stores. A useful tip especially for Londoners. Amex branches come in a creditable second in my experience and you might like to check out their rate for both TC's and wiring. Re the post office I find they are consistently beaten by a whole cent if you go the TravelCare/Travelshops chain which are run by the Co-Op.
As Charlie Zulu makes very clear: A bit of time spent shopping around at home could pay for an hour or two when you get there.
Please fill in the obvious blanks regarding slipping off around the world with great wodges of cash. Hole in the wall gives decent rate for day to day spending if you're off hour building Etc and credit card gives some measure of protection for bigger bills.
Regards
Rob