Is there anything wrong with the simple approach of just adding \<space> after the command? For example:
\newcommand{\T}{Mike}
\T\ is \T.
Which will give you:
Mike is Mike.
Input with "\newcommand{\gb}{Great Britain\xspace}":
\gb is a very nice place to live.\\
\gb, a small island off the coast of France.\\
\gb\footnote{The small island off the coast of France.}
is a very nice place to live.
Great Britain is a very nice place to live.
Great Britain, a small island o the coast of France.
Great Britain[^1] is a very nice place to live.