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My interpretation is:
In the broadest of terms the following applies to the reproduction of stamps in the UK and USA.
Stamps are subject to copyright. In many cases stamps are also trademarked (the Penny Black, for example). In order to reproduce them in any form you must apply to the postal service (Royal Mail/USPS) for a special licence relative to the application of the reproduction.
Dependent, there may be fees involved (primarily administration fees). The process will likely involve submitting for approval the design in which a reproduction is incorporated.
As stated, this is only my interpretation based on what I've learnt; the best thing to do is some homework:
Links (if allowed):
Royal Mail reproduction guidelines [royalmail.com].
USPS Licensing [usps.com].
Hope that's of some use.