Flash with Firefox on Linux respects the SOCKS proxy setting.

Indeed, Flash on OS X (at least, in Mountain Lion anyway) does not respect proxy settings. Additionally, the Flash BBC iPlayer will not work if it connects from the United States, despite what SG- has said below.

Which is more efficient if the sites don't care where the flash connection comes from (like, apparently, the BBC)

Actually, the Flash BBC iPlayer will _not_ work if it connects from the United States. Additionally, I discovered that the BBC is using Akamai for streaming their live content, and Akamai's United States servers do not have the content. So even if you convince the BBC that you're in England, but talk to Akamai servers in the US, the content does not stream.

Interestingly, BBC is using Limelight for pre-recorded content, and that content is apparently cached globally.

I never tried the live streams - the time difference was inconvenient enough.

Live content gets licensed differently to the catch-up content, which you can view without paying the license fee. Given catch-up is usually more restricted (some programmes unavailable for example) I imagine the only reason Limelight is globally accessible is because that's the default mode of operation and the iPlayer website is considered a sufficient region restriction mechanism.

flash in OSX will work just fine through a SOCKS connection that you setup in systems prefs -> network. however it won't work with just an HTTP proxy.

This is patently untrue, at least with the latest Flash in Mountain Lion. If you read the top paragraph of my post, you'd know this.

i don't know what to tell you but i'm doing it right now using an ssh proxy with ssh -D in ML with the latest flash (also on a SL machine next to me) to a VPS i have in UK (i'm from Canada).