First, make sure the font you're using has a £ sign in it.
Second, make sure you don't have Font Embedding enabled. If you have
fonts embedded and subset, Publisher uses the subset fonts rather than
the local system fonts, so any characters that were not present (such as
a £ sign) at the time of embedding are unavailable. Only way to clear
this out is to disable font embedding and re-enable it (although I don't
recommend using it anyway - it causes more trouble than it's worth).