As for the license itself -- even though I used to work on licensing and UI for Wikimedia Commons, I acknowledge this can be quite confusing, so I sympathize. I've rewritten this comment a couple of times already.
It's not clear to me if you can re-release everything under any single license. It's also unclear to me if you can assert copyright over the whole thing, as you must if you are going to use a CC license or the BSD-style license you used.
However, I would suggest that whatever you are doing, you should not arbitrarily reassign the work to a BSD license. They are not designed for graphics, since they require publication of the license wherever the graphic is used. Imagine if you wanted to use the icon on a postcard; according to the license you'd have to include the license text on the postcard. If you want these to be used widely Creative Commons Zero is much better... assuming you can assert copyright for the whole thing as a derivative work.
http://creativecommons.org/publicdomain/zero/1.0/
The really pedantically correct thing would be to list the licenses for each and every flag, but I acknowledge that would be no fun at all, and as far as I know Wikimedia Commons doesn't have tools to make that easy with large collections.
Sorry for the legalese - I think you did a great thing here, I'm just trying to help you share it with others.